The correct option is C follower
The word 'harbinger' refers to a forerunner of something.
Option A, precursor refers to a person or thing that comes before another of the same kind.
Option B, tender refers to being young, inexperienced or vulnerable.
Option C, opponent refers to someone who competes with or opposes in another.
Option D, follower refers to a person who moves or travels behind someone or something.
Thus, according to the meaning, the word 'follower' is the opposite of the word 'harbinger'.