Active hydrogen is:
a.) hydrogen with unit positive charge
b.) hydrogen with unit negative charge
c.) a atomic hydrogen
d.) a molecular hydrogen

Hint: Active hydrogen is so called because it is highly reactive. Active hydrogen can be used to convert active oxygen to water. It is a very strong reducing function.

Complete step by step solution:
Active hydrogen is the hydrogen that exists alone. It is an atomic hydrogen which exists as a single atom of hydrogen and not as a molecule of hydrogen. It is highly reactive in nature. And because it is highly reactive in nature, it is called active hydrogen.

Active hydrogen is an atom of hydrogen which is attached to a more electronegative atom making it highly polar. It also makes it unstable, which means highly reactive and as during the reaction it always leaves as ${H}^{+}$ ion which is commonly known as acidic hydrogen.
There are active hydrogen compounds, which means the compounds which can give a proton.

When hydrogen rich water enters our body, the molecular hydrogen is broken into active hydrogen. This active hydrogen bonds with the active oxygen, causing all kinds of diseases to produce safe and non-toxic in the form of water which leaves our body in the form of sweat and urine. This ultimately gets rid of the active oxygen that is produced in the body from moment to moment and reduces our chances of getting sick.
This shows that active hydrogen can be helpful to the human body.
Hence, option (c) is the correct answer.

Note: Do not confuse active hydrogen as the acidic hydrogen. Acidic hydrogen is the hydrogen with a positive charge $({H}^{+})$. Whereas, active hydrogen is just the atomic hydrogen or we can say that it is a single atom of hydrogen.