(d) Assertion (A) is false, but Reason (R) is true.
The correct procedure is to add acid slowly to water (not water to acid) with constant stirring, as dissolving acid in water is highly exothermic. Thus, Assertion (A) is false. Concentrated nitric acid is indeed soluble in water, so Reason (R) is true.
Source: Chapter 2, Section 2.2.1
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The key fact from the textbook is: "The acid must always be added slowly to water with constant stirring" — never water to acid. So the Assertion reverses this and is false. The Reason states nitric acid is soluble in water, which is true (all common mineral acids are water-soluble). Since A is false but R is true, the answer is (d). Watch out — examiners frequently test this dilution rule as it is a safety precaution explicitly highlighted in the textbook.