Concept Used: Concept of electric field 

Detailed Solution: 

Initally we say that, 

$\text{ Electric Force }=\text{ mg. }$

If electric field is switched off, there whould be net downwards gravitational force hence downward acceleration will there and it gain a velocity. 

If electric field is switched on, then force balance each other and acc become zero and it moves with constant velocity in downward direction.