Step 1: Check if the Assertion is true.
Both Sri Lanka and Belgium did struggle with ethnic and linguistic differences among their people, so the Assertion is correct.
Step 2: Check the Reason carefully.
The Reason claims Sri Lanka had a Tamil majority and Belgium had a French speaking majority. Actually it is the opposite. Sri Lanka's majority speaks Sinhala, and Belgium's majority speaks Dutch, mostly in the Flemish region. So the Reason is false.
Step 3: Decide the right option.
Since the Assertion is true but the Reason is false, this points us to a specific option, not the ones where both are true or both are false.
Hence, the correct answer is option (C), Assertion true but Reason false.