A hydrogen atom normally has:

• $1$ proton in its nucleus
• $1$ electron outside the nucleus
• $0$ neutrons in the most common form of hydrogen

So a neutral hydrogen atom can be represented as:

$$\text{Hydrogen atom} = 1\text{ proton} + 1\text{ electron}$$

The question says the hydrogen atom has lost an electron. Losing an electron removes the only electron hydrogen has:

$$1\text{ proton} + 1\text{ electron} - 1\text{ electron} = 1\text{ proton}$$

So the particle left behind is just the hydrogen nucleus, which contains:

$$1\text{ proton only}$$

Therefore, the correct answer is:

A) one proton only