What is an acid?

A.

A molecule with a pH of 7

B.

A molecule with a pH higher than 7

C.

A molecule that releases hydroxide ions when added to water

D.

A molecule that releases hydrogen atoms when added to water

Not (A) because a molcule with pH 7 is neutral eg, water

Not (B) because a molecule with pH >7 is basic/alkali which is the opposite of acid.

This leaves C and D.

Lets take one of the most commonly known acids, hydrochloric acid which has the formula HCl.

When acids are added to water they split up (dissociate).

HCl cannot split to give hydroxide ions but can give hydrogen atoms (which are actually hydrogen ions > H+)

so the answer is D