What happens in a reduction half-reaction?
elements gain electrons
elements lose electrons
oxygen is added
oxygen is lost

Option (a) is the correct answer.

Explanation:

A reduction-half reaction is defined as the reaction in which there occurs gain of one or more number of electrons by a specie or element.

Or, in a reduction-half reaction there occurs loss of oxygen if it is present in a reaction.

But in a reduction-half reaction loss of electrons will always take place.

For example,

Reduction-half reaction:

So here, oxidation state of copper is changing from +2 to 0. Also, loss of oxygen is occurring in the reaction equation.

Oxidation-half reaction:

Thus, we can conclude that in a reduction half-reaction elements gain electrons.

the answer is c.the valence of the atoms combine .