I'm going to give my own opinion on your questions, which is a bit different from others':
1. The sentence you give is fine but maybe not best. In English, the difference between a singular and plural noun to describe a general notion such as power is subtle. In general, a singular noun seems to suggest a grander or more poetic scale, while the plural will come across as more purely descriptive. Eg., "the tragedy of war" indicates the tragic quality of war itself, while "the tragedies of war" is more specific and may mean only the tragedies of a particular war. In your case, to say that "music has strange power" is a general statement that is perhaps slightly more idiomatic as "music has a strange power", to move, to inspire, etc. "Music has strange powers", although quite similar in meaning, begs the question of what those powers are; a similar construction is "the witch has strange powers", which is of course a totally different concept. In a context where you want to make a general statement, about art or the human soul, the singular "power" is probably better.
2. First I will address the choice for the blank: (b) is the only one that is idiomatic or ever used in this kind of context. As for singular vs. plural, this connects to your first question; just remember that most of the time, if you're looking for a general expression (the human eye in general, not any one person's eye), stick with the singular. "The", in a similar way, connotes a general quality more than a/an. For reference, there are many expressions that take similar constructions, like "as the crow flies"-- indicating geographic distance on a map (not "as a crow flies" or "as the crows fly"). You can extend this concept to phrases that actually can be put in the plural, such as "the bear hibernates in winter". "Bears hibernate in winter" is the most blunt, factual way to put it, like what you would read in a textbook. "The bear hibernates in winter", by contrast, has a more literary or poetic quality, generalizing from the singular bear to all bears. Where idioms are concerned, or where any kind of grand or expressive phrasing is desired, try the singular first. Now that that is out of the way, I want to address the point about "so bright as to be seen with". As some users have said, this is rather literary in style, but it is not at all wrong. Modern day spoken English tends away from "so...as to" in favor of "...enough as to" or "so...that" or something similar. Eg., "he ran so fast as to beat his brother by a full minute". This phrasing was colloquial (commonly spoken) 100 years ago but it is now on the margins. Today we would say, "he ran fast enough to beat his brother" or "he ran so fast that he beat his brother". In your sentence, however, it doesn't sound so out of place to me, maybe because the content of the sentence isn't very colloquial to begin with. For a documentary or a book, as another user said, this would be completely acceptable. In spoken English it would not be the most common, but it is still acceptable. One final thing. You didn't ask about this, but for completeness I'll just say that "by" is perhaps a slightly better choice than "with". I'm really splitting hairs here (making a distinction between two very similar things), but since your phrasing is a bit more formal and in good English, keep in mind that "by" is often more proper sounding when the sense is "through the faculty of". To give a guideline for with vs. by is difficult, but in some cases like yours, when in the passive voice I think, "by" is a little better. "The various flavors were perceived by his palate", "a wrong note can be picked out by a keen ear", etc. Hopefully that doesn't confuse you.
3. D-G are fine. I'll try to pick out some nuances for you. "Busy"-- probably the most natural choice, although there is some vague sense of redundancy in that both the person and his/her schedule are being called busy. "Tight"-- suggests that you barely have any time in between things; also a little bit business-speak, as if you have meetings all day long. "Crowded"-- a tiny bit unnatural to my mind, but people say this. Maybe more casual, also has a slight connotation that you're not the one responsible for it (the way the crowds just fill up by themselves). "Full"-- a plain way to say it. Probably as common as "busy". Also, C sort of works, but it sounds odd. "My schedule is heavy" sounds like a certain kind of slang, maybe what a surfer or younger guy would say.
