What is an equilibrant an object that has no net force acting on it? the reaction force in an action-reaction pair of forces? an added force that produces equilibrium? an object with forces equal in magnitude and direction?

The question seems to be what is an equilibrant force.

The answer is "an added force that produces equilibrium.

Here you have more insight:

an object that has no net force acting on it? This object indeed is in equilibrium but the object is not the equilibran force.

the reaction force in an action-reaction pair of forces?

the reaction force is not an equilibrant force. The reaction force exists always but equilibrium is only possible if the net force is cero.

an added force that produces equilibrium? this is the right  answer.