One(A) of the most(B) important aspects of an(C) organic farm are(D) sanitation.
This is a question on agreement in number between the subject and predicate. The subject of the sentence is the singular noun one, so the use of the plural verb are is wrong. The correct verb is is.
The correct sentence is:
One of the most important aspects of an organic farm is sanitation.
So, (D) is the answer.
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