(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
A is true: $5^n = 5 \times 5 \times \ldots$ has only 5 as its prime factor; for a number to end in 0 it must be divisible by both 2 and 5, but 2 is absent from the prime factorisation of $5^n$, so it can never end in 0.
R is true: 1 and 5 are indeed the only factors of 5 (definition of a prime).
Why R does NOT explain A: The correct explanation uses the Fundamental Theorem of Arithmetic — $5^n$ lacks the prime factor 2, so it cannot be divisible by 10. The statement about 5 having only two factors (1 and 5) is a general property of primes and does not directly explain why $5^n$ cannot end in 0. Hence option (b).