(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
Nitric acid is a strong oxidising agent; it oxidises the H₂ produced to water and itself gets reduced, so no hydrogen gas is evolved.
The key fact is that HNO₃ acts as an oxidising agent — it immediately oxidises any nascent hydrogen to water and is itself reduced (to oxides of nitrogen). This directly explains why H₂ is not evolved when Zn reacts with HNO₃, making R the correct and complete explanation of A. Choose option (a).