The correct option is A Forget
The word 'remember' refers to being able to bring some to one's mind in awareness.
Option A, think refers to directing one's mind towards someone or something.
Option B, realise refers to being fully aware of something as a fact.
Option C, forget refers to failing to remember.
Option D, recall refers to bringing a fact or event back to one's mind.
Thus, according to the meaning, the word 'forget' is the antonym of the word 'remember'.