Which of the following phenomena occurs, when a small amount of acid is added to water?

Correct option is B

Ionisation: When a small amount of acid is added to water, the acid molecules dissociate into ions (H+ and the anion of the acid) in water. This process is called ionization.
Dilution: As the acid is a small amount compared to the water, the overall concentration of the acid decreases. This decrease in concentration is called dilution.
Therefore, adding a small amount of acid to water results in both ionization and dilution.