aceleration of the block = qE/m

if it starts from rest, the velocity when it hits the wall = sqrt(2qEd/m)

since the collision is elastic, the block would return back with the same velocity, now because of the electric field it would de-accelerate...and finally stop after a displacement of d.

Now, the time period of oscillation is the time taken by the block to return....which can be easily calculated using eqns of motion..

for the block to hit the wall if time =t

then, t=v/a

= sqrt(2dm/qE)

so total time period = 2t = 2sqrt(2dm/qE)...(ans)