I will assume here that the well is sufficiently short so that the time the sound takes to come from the bottom of the well to our ear is negligible.
Since the pebble moves by uniformly accelerated motion, the distance it covers is given by
where
is the gravitational acceleration
is the time the pebble takes to reach the bottom of the well
Therefore, the depth of the well is
and the correct answer is B.
Since the pebble moves by uniformly accelerated motion, the distance it covers is given by
where
Therefore, the depth of the well is
and the correct answer is B.