A neutron of mass number 1 and kinetic energy 2.0 MeV hits a stationary nucleus of mass number a…

Step 1. Deuterium. $\dfrac{4 \times 2}{(2 + 1)^2} = \dfrac{8}{9} = 0.889$.

Step 2. Carbon-12. $\dfrac{4 \times 12}{(12 + 1)^2} = \dfrac{48}{169} = 0.284$.

Step 3. The energy left. $2.0\ \text{MeV} \times (1 - 0.284) = 1.43$ MeV.

Why light nuclei. For hydrogen ($a = 1$) the fraction is $\dfrac{4}{4} = 1$: the neutron can stop in one collision. For lead ($a = 208$) it is $\dfrac{4 \times 208}{209^2} = 0.019$, so the neutron loses at most $2\ \%$ of its energy in a collision. It would need thousands of collisions, and it would be lost on the way. This is why the moderator is made of light nuclei such as hydrogen in water, deuterium in heavy water, or carbon in graphite.