(d) Assertion (A) is false, but Reason (R) is true.
The Assertion is false — ventricles have thicker walls than atria. The Reason is true — ventricles pump blood to various organs at high pressure.
Source: Chapter 5, Section 5.4.1 (Transportation in Human Beings)
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The textbook clearly states: "ventricles have to pump blood into various organs, they have thicker muscular walls than the atria do." So the Assertion incorrectly swaps the relationship — it's the ventricles that are thick-walled, not the atria. The Reason correctly explains why ventricles need thick walls. Hence option (d) is correct.