(c) Assertion (A) is true, but Reason (R) is false.
The Assertion is correct — rainbow is a natural spectrum of sunlight. However, the Reason is false because a rainbow is formed opposite to the Sun (not when the Sun is overhead) after a rain shower.
Source: Chapter 10, Section 10.4 — Dispersion of White Light by a Glass Prism
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The key error in Reason (R) is the phrase "sun is overhead." The textbook clearly states that a rainbow is always formed in a direction opposite to that of the Sun. The sun being overhead is incorrect — the observer must have the Sun behind them. The presence of water droplets is correct, but since one part of the Reason is wrong, the whole Reason is false, making option (c) the right choice.