Question

A heavier unstable nucleus breaks into two fragments of comparable mass in a nuclear fission reaction and energy is released because:

• A. The total rest mass energy of the two fragments is less than the rest mass energy of the heavy nucleus.
• B. The binding energy per nucleon of the two fragments is more than the binding energy per nucleon of the heavy nucleus.
• C. The nuclei of the fragments are more bound than the nucleus of the parent.
• D. NONE OF THE ABOVE

Answer 1

Answer: (A), (B), (C)

The nuclei of the fragments are more bound than the nucleus of the parent.

Since we know that the energy is released, means it is given out, that the reaction is exothermic.
It has a direct implication that the rest mass energy of the parent must be more than the rest mass energy of the daughters.
​$X\to Y+Z$
Here Rest mass energy of X > Rest mass energy of Y + Rest mass energy of Z.
The released energy comes out eventually as heat.

Now, looking at the Binding energy per nucleon graph we observe that in any spontaneous fission reaction, the B/A value of the lighter fragments is more than the B/A of the heavier parent.
It's simple to understand right! Since the energy has been released, the binding energy of Y and Z must increase.
Also the increased Binding energy implies that the fragments are more bound than the parent.
So all the three options are correct.