(c) Assertion (A) is true, but Reason (R) is false.
A is true — humans show great variation. R is false — not all variations have equal survival chances; environment selects certain variants over others.
Source: Chapter 8, Section 8.1 — Accumulation of Variation During Reproduction
The textbook explicitly states: "Do all these variations in a species have equal chances of surviving in the environment in which they find themselves? Obviously not." Environmental selection favours certain variants (e.g., heat-resistant bacteria survive a heat wave). So the Assertion is correct, but the Reason directly contradicts the textbook — making option (c) the right choice.