The sentence that uses parentheses correctly is:
C The apartment (which hadn't been lived in for a full year) was filled with trash and debris.
In this sentence, the parentheses are used properly to include additional information without interrupting the main sentence.
The other sentences have issues with parentheses placement or punctuation:
A - The parentheses are not placed properly; they should be at the start of the sentence, not before "our."
B - The parentheses also should not be at the beginning or redundant at the end of the sentence.
D - The parentheses are incorrectly placed; there should be a closing parenthesis before "isn't."