What is a reduction reaction? A. A reaction in which an atom forms a new isotope by gaining neutrons.
B. A reaction in which charged particles form an ionic compound.
C. A reaction in which negatively charged particles are in the products.
D. A reaction in which an atom has gained 1 or more electrons.

D. A reaction in which an atom has gained 1 or more electrons.

Explanation:

The oxidation reduction reactions are called redox reaction. These reactions are take place by gaining or losing the electrons and oxidation state of elements are changed.

Oxidation:

Oxidation involve the removal of electrons and oxidation state of atom of an element is increased.

Reduction:

Reduction involve the gain of electron and oxidation number is decreased.

Consider the following reactions.

4KI + 2CuCl₂  →   2CuI  + I₂  + 4KCl

The oxidation state of copper is changed from +2 to +1 so copper get reduced by gaining the one more electron

let that person have a turn to speak : )