Assertion :An object falling from a great height may reach a steady terminal velocity.
Reason :The viscous force on the body is responsible for this steady terminal velocity.

An object falling from a great height first accelerates due to gratuity but as its velocity increases, the viscous force increases, this keep on increases to a point where the viscous and buoyant  force balances the weight of the body and thus a terminal velocity is reached.
The equilibrium equation,
$6\pi \eta rv+v{\rho }_{air}g=V\rho g$
(viscous force)(buoyant force) (weight of the body)