The isotope of hydrogen referred to in the question is not explicitly mentioned, so we will consider all possible isotopes of hydrogen to address the query properly. Hydrogen has three isotopes: protium ($^1H$), deuterium ($^2H$), and tritium ($^3H$). The sum of the number of neutrons and protons (nucleons) in each isotope gives us:

• Protium ($^1H$): It has 1 proton and 0 neutrons, so the sum is $1 + 0 = 1$.
• Deuterium ($^2H$): It has 1 proton and 1 neutron, so the sum is $1 + 1 = 2$.
• Tritium ($^3H$): It has 1 proton and 2 neutrons, so the sum is $1 + 2 = 3$.

Among the provided options, Option D: 3 is correct for tritium, the third isotope of hydrogen. Therefore, the sum of the number of protons and neutrons in the tritium isotope of hydrogen is 3.