(C) (A) is true, but (R) is false.
Assertion is correct — both countries faced ethnic/linguistic diversity. But the Reason is false: Sri Lanka had a Sinhala majority (74%), not Tamil majority; Belgium had a Dutch-speaking majority (59%), not French-speaking majority.
The key error in Reason (R) is that it reverses the majorities. The textbook clearly states Sinhalas are 74% in Sri Lanka and Dutch-speakers are 59% in Belgium. Examiners expect you to identify this factual mistake to choose option (C). Always re-check which community holds the majority in each country.