The requirement that a reversible reaction be at equilibrium is that:

a, the concentrations on the two sides of the arrow be equal

b. the velocity for the forward reaction equal that of the reverse reaction

c. there will be as many molecules of the substances on one side of the arrow as

there are molecules of the substances on the other side

d. the moles of products will equal the moles of reactants

B.the velocity for the forward reaction equal that of the reverse reaction

Explanation:

A state of rest or motion of a system is described as equilibrium.

For a system to be in equilibrium, the following conditions must be met:

1. It must be involved in reversible chemical process no matter how small the extent of the reversibility.

2. The rate of forward process is equal to the rate of backward or reverse process.

3. The system must be closed.

4. There is no change in the concentration of each of the species in equilibrium with respect to time. There must be a constancy of concentration of each species in the equilibrium process.

When we change any of the conditions of equilibrium, the Le Chatelier's principle comes into play.

The principle states that "if any of the conditions of a system in equilibrium is changed, the system will adjust itself in order to annul the effect of the change".

B.the velocity for the forward reaction equal that of the reverse reaction

Explanation:

A reversible reaction proceeds in a closed system. Equilibrium is only attainable in a closed system. When a reaction occurring in a closed system attains equilibrium, the rate(velocity) of the forward reaction will be exactly the same as the rate (velocity) of the reverse reaction.

This implies that the forward and reverse reactions proceed at the same rate.

the answer would be c