The momentum of the movement slackened in course of time.

Answer: Option C

Slackened : reduce or decrease in speed or intensity.
Recovered : return to a normal state of health, mind, or strength.
Multiplied : increase or cause to increase greatly in number or quantity.
Quickened : make or become faster or quicker.
Stopped : (of an event, action, or process) come to an end; cease to happen.

Antonym of Slackened is Quickened