Assertion: A solution containing a mixture of acetic acid and sodium acetate maintains a constant value of pH on the addition of small amounts of acid or alkali.

Reason: A solution containing a mixture of acetic acid and sodium acetate acts as a buffer solution. 

1.  Both the assertion and the reason are true and the reason is the correct explanation of assertion.

2.  Both the assertion and the reason are true and the reason is not the correct explanation of assertion.

3. The assertion is true but the reason is false.

4.  The assertion is false but the reason is true.

Hint: Buffer solution resist the pH change
A solution containing a mixture of acetic acid and sodium acetate acts as a buffer solution as it maintains a constant pH which is not affected by the addition of small amounts of acid or alkali.