Which isotope of hydrogen has the same number of protons and neutrons in its nucleus? 

Deuterium

The correct answer is option 2. Deuterium.

Hydrogen is a unique element because it has three isotopes: protium, deuterium, and tritium. Each isotope of hydrogen differs in the number of neutrons present in its nucleus while maintaining the same number of protons (which defines it as hydrogen).

1. Protium \((^1H)\):  This is the most common isotope of hydrogen and consists of only a single proton in its nucleus, with no neutrons. Protium is by far the most abundant isotope of hydrogen, making up about 99.98% of naturally occurring hydrogen.

2. Deuterium \((^2H)\): Deuterium, also known as heavy hydrogen, has one proton and one neutron in its nucleus. This gives it a mass number of 2. Deuterium is stable and accounts for a very small percentage of naturally occurring hydrogen, about 0.02%.

3. Tritium \((^3H)\): Tritium is a radioactive isotope of hydrogen with one proton and two neutrons in its nucleus, giving it a mass number of 3. Tritium is unstable and decays over time, emitting beta particles. It is rare in nature and usually produced artificially for various applications.

Among these isotopes, deuterium is the one that has the same number of protons and neutrons in its nucleus, making it unique in this regard. This balanced proton-neutron composition contributes to its stability compared to tritium, which is radioactive, and protium, which lacks neutrons.