Option A — Both (A) and (R) are true and (R) is the correct explanation of (A).
Belgian leaders recognised regional differences and cultural diversities, which directly led them to design power-sharing arrangements accommodating linguistic divisions.
The passage explicitly states: "They recognised the existence of regional differences and cultural diversities" and then describes how they amended the constitution to create power-sharing arrangements. R directly causes A, so R is the correct explanation of A. Choose Option A, not B.