The correct option is A quiet
The word 'babel' refers to a confused noise made by a number of noises.
Option A, quiet refers to making little or no noise.
Option B, din refers to a loud, unpleasant and prolonged noise.
Option C, mourn refers to feeling regret or sadness about something.
Option D, cry refers to shouting or screaming, typically to express fear, pain or grief.
Thus, according to the meaning, the word 'quiet' is the opposite of the word 'babel'.