1. Key physical principle

For a nuclear reaction, energy is released when the total binding energy increases.

If the products have larger binding energy than the reactants, then

Since binding energy per nucleon is plotted, the total binding energy is approximately

where $A$ is mass number.

Also, from the standard binding energy per nucleon curve:

• Light nuclei gain energy by fusion if they move toward higher $E_b$.
• Very heavy nuclei gain energy by fission if they split into nuclei with higher $E_b$.

2. Interpretation of the graph

Although the figure is not shown here, this standard question uses the usual binding-energy-per-nucleon curve:

• $A,B,C$ lie in the lighter side region where combining lighter nuclei can move to a nucleus with larger $E_b$.
• $D,E,F$ lie in the heavier side region where splitting a heavy nucleus can move to nuclei with larger $E_b$.

So we test each reaction qualitatively.

3. Reaction (i):

This is fusion of lighter nuclei to form a heavier nucleus $C$.

For light nuclei, fusion releases energy if the product has higher binding energy per nucleon than the reactants. From the binding-energy curve, this is true.

Hence,

for (i).

4. Reaction (ii):

This is the reverse of reaction (i). If (i) releases energy, then the reverse must require energy input.

So for (ii), energy is not released:

Thus (ii) is not positive.

5. Reaction (iii):

This is fusion on the heavy side of the binding-energy curve.

For heavy nuclei, fusion generally moves to lower binding energy per nucleon, so it does not release energy.

Hence for (iii),

so (iii) is not positive.

6. Reaction (iv):

This is fission of a heavy nucleus. On the heavy side, splitting into medium-mass nuclei increases binding energy per nucleon, so energy is released.

Therefore,

for (iv).

7. Final selection

The reactions with positive energy release are:

So the correct option is