1a) What you need is a lot of practice/practise.
1b) All you need is a lot of practice/practise.
1c) All you have to do is a lot of practice/practise.
They could all be used, but I think A and B are more common and more natural than C. The problem with C is that it has 11 syllables, but A and B only have 9, so it's faster to say A and B. The difference is small enough that one might be more common than another in some places or even just depending on personal preference.
2a) What you need is to practice/practise a lot.
2b) All you need is to practice/practise a lot.
2c) All you have to do is to practice/practise a lot.
It's hard to say if these are more common than the first set or not. I feel like the way the word "to" is being used makes these sound a little more forceful than the first set. It seems more like a command is being given. I think that grammatically the second "to" in C isn't needed at all.
3a) What you need is practice/practise a lot.
3b) All you need is practice/practise a lot.
3c) All you have to do is practice/practise a lot.
I think C is the only one that sounds correct. A and B are both missing a preposition to connect "need" with "practice." In this case the use of "to" doesn't sound as forceful to me.
Out of all of these 3c sounds like the one I would be most likely to use, but again that's just my personal preference.
4a) This is the novel I was telling you about the other day.
4b) This is the novel that/which I was telling you about the other day.
4c) This is the novel about which I was telling you the other day.
For B it's very complicated because "that" and "which" have different uses and can't be used in place of each other with the same meaning. I immediately thought "that" sounded correct and "which" incorrect, but then I looked it up to be able to explain why and I discovered a confusing world of subordinate clauses, restrictive clauses, and relative pronouns. I don't think ordinary people would know what these are, we just say what sounds right. I think "that" is correct here.
"That" is used for restrictive clauses and "which" is used for non-restrictive clauses. The difference is that if you removed a non-restrictive clause from a sentence then you would still have a sentence with the same meaning, but a restrictive clause is important to the meaning of the sentence and can't be removed without changing the meaning.
So if you remove "which I was telling you about the other day" from the sentence then all you would be left with is "This is the novel." I think that changes the meaning of the sentence, because now the meaning of the sentence is lost, so it was a restrictive clause and the word "that" should be used.
I'm completely sure that the word "that" is what should be used, but I'm not 100% sure that I'm explaining it correctly.
I think A is the most natural because it has the fewest unnecessary words. To be grammatically correct I think it needs the word "that" but I hear people talk like that often. I think C is correct, but it sounds like it might be an old fashioned way of saying it. At least that's how it might sound in America. Maybe it's used in Britain?
5a) I want a house with a large garden.
5b) I want a house that/which has a large garden.
5c) I want a house whose garden is large.
A sounds completely natural and correct. B should use the word "that" and it could also be used. C would only be correct if you were talking about a person, but a house is not a person so the word "whose" would not be used.
10a) New York is the only city I have visited in the U.S.
10b) New York is the only city that I have visited in the U.S.
10c) New York is the only city which I have visited in the U.S.
10d) New York is the only city I have ever visited in the U.S.
10e) New York is the only city that I have ever visited in the U.S.
10f) New York is the only city which I have ever visited in the U.S.[/I]
6a
The use of "whose" is incorrect here.
6b - 8a
These are correct.
8b
The use of "which" here is incorrect because the clause that follows it is very important to the meaning of the sentence.
9a
This sounds natural.
9b
I don't know if the word "that" is needed.
9c
This is correct.
9d
The use of "whom" is incorrect because "the American man" is the subject of the sentence. "Whom" would be used if he was the object of a sentence.
10a and 10b
Again, I'm not sure if the word "that" is needed to be completely grammatically correct, but to me it sounds okay to not have it. It's probably better to use it.
10c
The use of "which" here is also incorrect because if you remove the clause that follows it then you are only left with "New York is the only city" and by itself it doesn't have the meaning of the full sentence.
10d and 10e
same as 10a and 10b.
10f
same as 10c
Other than the few corrections I noted above, all of the sentences from 6a - 10f sound completely natural.