Assertion (A): Acetic acid has four hydrogen atoms in its molecule but its basicity is one.

Reason (R): Acetic acid has only one replaceable hydrogen atom.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Both A and R are true and R is the correct explanation of A.

Reason — The molecule CH₃COOH possesses four hydrogen atoms. Three are attached to the methyl group (CH₃) and are non-ionisable, while the single hydrogen in the –COOH (carboxylic) group is ionisable. Because only one hydrogen can be released as an H⁺ ion, the basicity of acetic acid is 1. Thus both the Assertion and the Reason are correct, and the Reason properly explains the Assertion.