Question

which reaction represents fusion?

Explanation:

Nuclear fusion is a reaction where two small atomic nuclei combine to form a larger nucleus.

1. The top reaction is nuclear fission (a heavy nucleus splits).
2. The blue card reaction shows two small hydrogen nuclei ($^2_1\text{H}$) merging to form a larger helium nucleus ($^4_2\text{He}$), which matches fusion.
3. The orange card reaction is alpha decay (a nucleus emits an alpha particle).
4. The bottom reaction is neutron capture with decay, not fusion.

Answer:

$\boldsymbol{^2_1\text{H} + ^2_1\text{H}
ightarrow ^4_2\text{He}}$ (the reaction on the blue card)