While the vast majority of hydrogen atoms consist of one proton and one electron, there are isotopes that have neutrons as well:

${?}^{2}H($also called deuterium$)$ has one neutron along with its proton, and has a mass of $2\mathrm{.}014102$ u $.$

${?}^{3}H($also called tritium$)$ has two neutrons along with its proton, and has a mass of $3\mathrm{.}016049$ u $.$

Regular ${?}^{1}H$ has zero neutrons, and has a mass of $1\mathrm{.}007825$ u $.$

If you were going to make a helium atom out of hydrogen, what combination would release the most energy? For reference:

${?}^{3}He($two protons $+$ one neutron$)$ has a mass of $3\mathrm{.}016029$ u $.$

${?}^{4}He($two protons $+$ two neutrons$)$ has a mass of $4\mathrm{.}002602$ u $.$

${?}^{1}H$ and ${?}^{2}H$ combining to ${?}^{3}He$ would release the most energy.

${?}^{1}H$ and ${?}^{3}H$ combining to ${?}^{4}He$ would release the most energy.

${?}^{2}H$ and ${?}^{2}H$ combining to ${?}^{4}He$ would release the most energy.

Either way of making ${?}^{4}He$ would release the same amount of energy, and more than the ${?}^{3}He$ process.

All three processes would release the same amount of energy.