(A) : If a heavy nucleus is split into two medium sized parts, each of the new nuclei will have more binding energy per nueleon than the original nucleus.
(R) : Joining two light nuclei together to give a single nucleus of medium size means more B.E per nucleon in the new nucleus.

In fission reaction, heavier unstable nucleus splits in daughter nuclei with higher binding energy per nucleon, which gives rise to energy emission. 

In fusion reaction, lighter nuclei with lower binding energy per nucleon fuse to form a heavier nucleus with higher binding energy per nucleon, which gives rise to energy emission.

Both (A) and (R) are true and (R) is not the correct explanation of (A).