(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
Belgium amended their constitution four times (1970–1993) specifically to work out arrangements to enable everyone to live together within the same country.
The passage directly states: "they amended their constitution four times so as to work out an arrangement that would enable everyone to live together within the same country." Both statements are factually correct, and R directly explains why A happened — making (a) the right choice.