(d) (A) is false but (R) is true.
Sri Lanka adopted Sinhala (not Tamil) as the only official language in 1956, disregarding Tamil. However, the Government of Sri Lanka did adopt majoritarian measures to establish Sinhala supremacy, so (R) is correct.
Source: Majoritarianism in Sri Lanka, Chapter 1
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The key fact to remember: the 1956 Act made Sinhala the only official language — Tamil was disregarded, not recognised. Assertion (A) swaps the two languages, making it false. Reason (R) is accurate — the Sri Lankan government did adopt majoritarian measures. This is a classic "fact-swap" trap in Assertion-Reason questions; always verify each statement carefully against the textbook.