C) Kale, which is nutritious and low in fat, has become available in greater variety.
Explanation:
Compound senteces are made out of two independent sentences that have related ideas and that are joint together by a conjuction or a semicolon, in this case A, B, and D, have conjuction uniting the sentences, And-But, are conjuctions often used to combine compound sentences, in the case of C the sentence is only using a comma to separate ideas. So C is not a compound sentence.
