Iru Function Here

Hello again~
I was doing a Japanese practice exam and the first thing said before the questions start is this sentence:

次の__に入るもっとも適当なものをA~Dの中から一つ選びない。

I can look at it and know immediately what it means, but to actually build it up myself or translate it exactly how it is, I cannot do that.

That Iru is what is really getting to me.

First of all, it means "To Set/To Set In" so I don't know how it is describing Mottomo or why it is before the Wo. Or is it just that the 次の__に入る is kind of like a little sentence inside of the entire sentence?

Basically I know that the part in Green is where the selected answer will go.
The part in Blue is the part describing the answer which is "Most suitable".
And the Orange part is saying "Please choose one answer from A~D."

I can gather that 入るもっとも適当なもの is the object of 選ぶ, however. I've looked up examples of it already also but none of them seem to match up with the way it is used here. So, if you could please explain then that would be great!

Oh and my translation for this is:

Please select the most suitable word from A~D to set in the following blank.
Thank you!

選びなさい

もっとも適当な is a な adjective and it modifies もの

入る means insert/put

次の__に入る means insert/fill in the following blank/space

Also no ''please'' here. なさい is a command.

Thank you!

次の__に入る modifies もっとも適当なもの. 入る is intransitive, so the literal translation is "Choose the most appropriate one that can be put in the following blank among A~D".

The reading is はいる, not いる, BTW.