The Question Stem

When the question stem provides a brief question that touches upon a sentence in the passage, is it referring back to the entirety of the sentence or only the phrase provided? For instance, the question stem includes "zooplankton feeds on phytoplankton" (or something like that) but doesn't include the rest of the sentence. When we answer the question, are we addressing the answer choices as if only the phrasing given in the question stem is what the question is referring to, or are we approaching the answer choices as if the entirety of the sentence that includes the above-mentioned phrase is what the question stem is referring to? I hope that makes sense.