(b) Both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A).
A is true: Tangents at endpoints of a diameter are perpendicular to the diameter (Theorem 10.1), so they are parallel to each other (both perpendicular to the same line).
R is true: Theorem 10.2 states tangents from an external point are equal in length.
R does not explain A: The parallelism of the tangents follows from the radius-tangent perpendicularity (Theorem 10.1), not from the equal-length property. So (b) is correct.