Final answer:
A reaction that involves the gain of one or more electrons is known as a reduction reaction, which is part of the broader category of oxidation-reduction (or redox) reactions.
Step-by-step explanation:
A reaction involving the gain of one or more electrons is a reduction reaction. In chemistry, reactions where there is a transfer of electrons are called oxidation-reduction (or redox) reactions.
Oxidation is characterized by the loss of electrons, gain of oxygen, or loss of hydrogen, while reduction is characterized by the gain of electrons, loss of oxygen, or gain of hydrogen. Oxidation and reduction always occur simultaneously as part of redox reactions. Therefore, the correct answer to the question is B) reduction.