A mass attached to a string that is itself attached to the ceiling swings back and forth. a$.)$ At the bottom of the arc, it has no net force acting on it$.$

b$.)$ At the bottom of the arc, it has no net force acting in the vertical.

c$.)$ At the bottom of the arc, tension and gravity balance one another out.

d$.)$ At the bottom of the arc, there is no centrifugal force.