A hydrogen atom which has lost an electron contains

Correct answer: one proton only.

Explanation: A neutral hydrogen atom (the common isotope, protium) has 1 proton and 1 electron, with no neutron. If it loses its electron, it becomes H⁺—which is simply a single proton.

• one neutron only – Incorrect. Protium has no neutron.
• one proton and one neutron – Incorrect. That would be deuterium’s nucleus, not ordinary hydrogen after ionization.
• one proton, one electron and one neutron – Incorrect. The electron has been lost and protium has no neutron.