Answer:
C.
Step-by-step explanation:
Since explicit is defined as "fully and clearly expressed," the sentence that most directly states against this, or the antonym of explicit, is going to be the answer.
In choice A, it states that the person was frank in stating how he felt. Frank is defined as "clear." Therefore, it is a synonym of explicit and not an antonym.
In choice B, it states that the directions were clear. So that could not be the answer.
In choice C, it states that the effects of the exercise program were "indirect." Now, if we look back to the definition of explicit, explicit means "fully and clearly expressed." The word "inderect" is the opposite of "explicit." Therefore, the answer is C.