The correct option is B reject
The word 'accept' refers to consenting to receive or undertake.
Option A, deny refers to refusing to give something requested to someone.
Option B, admit refers to confessing to be true or to be the case.
Option C, reject refers to dismissing as inadequate, unacceptable of faulty.
Option D, decline refers to becoming smaller, fewer or less.
Thus, according to the meaning, the word 'reject' is the opposite of the word 'accept'.