Question

give reason for the decrease of binding energy for nuclei with high mass number​

Answer 1

Fusion is the combining of light elements to form heavier elements since the heavier elements have higher binding energy per nucleon, therefore more stable. As the number of nucleon reach 60 the energy released become less and less.

Fission is the splitting of the nucleus of a heavy element to smaller and lighter elements which have higher binding energy per nucleon.