The element common to all acids is A. Hydrogen B. Carbon C. Sulphur D. Oxygen

The correct answer is Hydrogen.

An acid is a substance that donates hydrogen ions (H+) in a solution. The element common to all acids is hydrogen. This is because the hydrogen ion is the part of the acid that is responsible for its acidic properties.

The other options are not common to all acids. Carbon is found in some acids, such as acetic acid (CH3COOH), but not in all acids. Sulphur is found in some acids, such as sulphuric acid (H2SO4), but not in all acids. Oxygen is found in some acids, such as nitric acid (HNO3), but not in all acids.

Therefore, the correct answer is Hydrogen.