Option (B): Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
Mazzini was indeed an Italian revolutionary, and he did found underground societies (Young Italy, Young Europe). However, founding underground societies does not explain why he was a revolutionary.
The examiner wants students to distinguish between a reason being true and it being the correct explanation. Here, both facts are correct (sourced from the textbook), but founding secret societies is a consequence/activity of being a revolutionary, not the explanation for it. Option B is the standard answer for such cases.