When the drag force acting on a swimming object completely balances the object's propulsive force (be it in internal force, like the bacteria, or an external force, like gravity acting on the beads), the object reaches a

a. contant acceleration.
b. terminal speed.
c. comes to a stop.
d. constantly changing speed.

b. terminal speed.

Explanation:

When the drag force acting on a swimming object is completely balances the object's propulsive force then the according to Newton's first law of motion the body will continue to move with a uniformly constant velocity.The acceleration of the body will be zero as the net force acting on the body is zero. The speed hence attained as a result of balanced force due to drag on a moving body in the fluid is called terminal velocity.Drag force is similar to the frictional force in fluid when a body moves through a fluid.

26.4 is about the right time