SAT Question 6fdddabf: Boundaries answer and explanation

Choice B is the best answer. The convention being tested is punctuation use between a verb and its complement. No punctuation is needed between the verb “is called” and its complement “the chromophore.” The complement helps complete the idea of the verb—in this case, it explains what the part of a compound that determines the compound’s color is called—and any punctuation between the two results in an ungrammatical sentence.