The mass of hydrogen is 1.6726 x 10–27 kg, and the mass of helium is 6.6465 x 10–27 kg. Given this, which of the following nuclear reactions would result in a decrease of total mass, and thus a release of energy, while keeping the same number of particles involved?

A. A helium nucleus breaks up into four hydrogen nuclei.
B. Four hydrogen nuclei combine into one helium nucleus
C. Two hydrogen nuclei combine into one helium nucleus.
D. A helium nucleus breaks up into two hydrogen nuclei

B.Four hydrogen nuclei combine into one helium nucleus.

Explanation:

Mass energy which is stored in a matter itself can be very powerful. A small amount of mass can result in a very large amount of energy as the speed of light is very large.

If we consider the two elements hydrogen and helium, a hydrogen nucleus is made of one proton  and a helium nucleus is made of two protons and two neutrons.

The mass of hydrogen is 1.6726 x 10–27 kg, and the mass of helium is 6.6465 x 10–27 kg.

Four hydrogen nuclei combine into one helium nucleus would lead to decrease of total mass, and thus a release of energy such that the number of particles involved remains same.