(A) Both (A) and (R) are correct and (R) is the correct explanation of (A).
The civil war arose from failed political accommodation of ethnic (Sinhala-Tamil) differences, and the resulting distrust between the two communities directly caused the widespread conflict.
Both statements are factually correct as per the textbook. The Reason directly explains the Assertion — majoritarian policies led to distrust between Sinhala and Tamil communities, which escalated into civil war. So (R) is the correct explanation of (A), making option (A) the right choice.