(a) Both, (A) and (R) are correct and (R) is the correct explanation of (A).
The French-speaking minority was rich and powerful, and Dutch speakers resented this as they got economic development and education much later — R directly explains A.
The passage explicitly states: "The minority French-speaking community was relatively rich and powerful. This was resented by the Dutch-speaking community who got the benefit of economic development and education much later." This makes both A and R true, and R is the direct cause-and-effect explanation for A.