Assertion : A very heavy nucleus, say A = 240, has lower binding energy per nucleon compared to that of a nucleus with A = 120.

Reason : If a nucleus A = 240 breaks into two A = 120 nuclei, nucleons get more tightly bound.

Both (A) and (R) are true and (R) is the correct explanation of (A).

A very heavy nucleus, say A = 240, has lower binding energy per nucleon compared to that of a nucleus with A = 120. Thus if a nucleus A = 240 breaks into two A = 120 nuclei, nucleons get more tightly bound. This implies energy would be released in the process.

More tightly bound nucleons implies more binding energy per nucleon and more stability.