(d) Assertion (A) is false but Reason (R) is true.
Since $4^n = 2^{2n}$, its only prime factor is 2, never 5. So $4^n$ never ends with 0 — Assertion is false. Reason is true: if $x$ has both 2 and 5 as prime factors, then $x^n$ is divisible by 10, so it always ends with 0.
Source: Chapter 1, Section 1.2 (Example 1)
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The textbook (Example 1, Section 1.2) explicitly states that $4^n = (2)^{2n}$ contains only the prime 2, so it can never end in 0. This makes Assertion (A) false. The Reason correctly states a general principle — a number with both 2 and 5 as factors will always produce a multiple of 10 when raised to any natural number power — so Reason (R) is true. When A is false and R is true, the answer is option (d).