Assertion (A) states that if \( f(x) \) is not continuous at \( x = a \), then it is not differentiable at \( x = a \). This is true because for a function to be differentiable at a point, it must first be continuous at that point.
Reason (R) states that if \( f(x) \) is differentiable at a point, then it is continuous at that point. This is also true because differentiability implies continuity.
Thus, both assertion and reason are true, and reason correctly explains the assertion.