The correct option is
C Accept
The word 'contest' refers to engaging in competition to attain a position of power.
Option A, accept refers to consent to receive or undertake something offered.
Option B, defeat refers to winning a victory over someone in a battle or contest.
Option C, lose refers to failure in winning a battle or contest.
Option D, run for refers to attempting to become elected for a public position.
Thus, according to the meaning, the word 'accept' is the most opposite in meaning from the word 'contest'.