The correct option is B leave
The word 'undertake' refers to promising to do a particular thing.
Option A, refrain refers to stop oneself from doing something.
Option B, leave refers to go away from something.
Option C, conceal refers to being not allowed to be seen.
Option D, retrieve refers to getting or bringing back from somewhere.
Thus, according to the meaning, the word 'leave' is the opposite of the word 'undertake'.