SAT Question 7f48b098: Boundaries answer and explanation

Choice C is the best answer. The convention being tested is the punctuation of a supplementary element within a sentence. The comma after “nutrients” pairs with the comma after “photosynthesis” to separate the supplementary element “the mechanism by which plants use sunlight to turn carbon dioxide and water into nutrients” from the rest of the sentence. This supplementary element functions to define the term “photosynthesis,” and the pair of commas indicates that this element could be removed without affecting the grammatical coherence of the sentence.