Viscosity And Stokes' Law

According to NCERT Class 11, Chapter 10 (Mechanical Properties of Fluids), a body falling through a viscous fluid experiences three forces: its weight acting downward, the upward buoyant force, and the upward viscous drag described by Stokes' law, $F=6\pi \eta rv$. As speed increases the drag grows until the net force becomes zero; thereafter the body moves with constant velocity called the terminal velocity, given by $v_t=\frac{2r^2(\rho -\sigma )g}{9\eta }$. The option 'escape velocity' is wrong because that concerns gravitational escape from a planet, not fluid motion. The option 'critical velocity' refers to the speed at which flow changes from laminar to turbulent, a different concept. The option 'drift velocity' belongs to charge carriers in a conductor and is unrelated to a falling ball. A sanity check confirms that once acceleration ceases the velocity is steady, which is exactly the defining feature of terminal velocity, making it the only physically correct choice.