(b) Both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation for Assertion (A).
A is true: The tangent is perpendicular to the radius at the point of contact (Theorem 10.1). At both ends of a diameter, the radii point in opposite directions, so both tangents are perpendicular to the same line (the diameter), making them parallel.
R is true: The diameter is indeed the longest chord of a circle.
R does not explain A: The parallelism of the tangents follows from Theorem 10.1 (tangent ⊥ radius), not from the diameter being the longest chord. Hence option (b).