Question

a rain drop continues to fall with a uniform velocity when its weight is balanced by

Answer 1

when it's weight is balanced by the upthrust due to air

Answer 2

Weight of rain drop is balanced by frictional force (F) due to air. It’s also called viscous Force.

By Stokes law force (F) is given
F = 6πηrv

Where,
η = coefficient of viscosity
r = radius of raindrop
v = Velocity of rain drop