Questions: The isotope deuterium has one proton and one neutron.

The answer is the second one: one proton and one neutron.

Explanation for each option:

1. One proton: This describes the most common isotope of hydrogen, known as protium, which has one proton and no neutrons.
2. One proton and one neutron: This correctly describes deuterium, an isotope of hydrogen. Deuterium has one proton and one neutron in its nucleus.
3. One proton and two neutrons: This describes tritium, another isotope of hydrogen, which has one proton and two neutrons.
4. One proton and three neutrons: This does not describe any common isotope of hydrogen.

Summary: Deuterium, an isotope of hydrogen, has one proton and one neutron in its nucleus.