Acetic acid is a weak acid because:

The correct option is C It gets weakly ionised in water.
Acetic acid (${\text{CH}}_3\text{COOH}$) ionise partially in water, hence, it is a weak acid. In other words, at any time in the solution, only about some percentage of the weak acid molecules have ionised into corresponding ions, while the rest remain as simple acid molecules in water.