The correct option is A rarely
The word 'often' refers to something that occurs frequently or many times.
Option A, rarely refer to something that does not happen often.
Option B, never refers to occurring at no time in the past or future.
Option C, always refers to occurring at all times.
Option D, sometimes refers to something occurring occasionally.
Thus, according to the meaning, the word 'rarely' is the opposite of the word 'often'.