Either brad or ann (takes,take) out the garbage every night

Either brad or Ann takes out the garbage every night.

Takes is saying that it is an action that is repeated.

Take is a one time action in the present tense, or future.

"John will take out the trash"

You need a slight stop there to collect your thoughts, especially when the next word is achromatopsia. That makes D wrong.

If you use B, " A medical condition that limits her vision" is not a sentence. So B shouldn't be the answer. Fragments are used by very good writers usually in fiction, and this statement is not part of something that is fictional.

A semicolon separates two equal clauses. You can think of it as meaning "and" or even "but" sometimes. You have the same problem with C as you do with B. So C is not correct.

That leaves us with A. That should be your answer.You need a pause of some kind and that pause is proved by the comma. The sentence is not a run on sentence just one in need of a pause in the middle.