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.

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.