Which of the following species is not an isotope of hydrogen?

${}_1{}^4\mathrm{H}$ is not an isotope of hydrogen.

Isotopes are atoms of the same element having the same atomic number, but a different mass number. Hydrogen has three isotopes. They are protium ${}_1{}^1\mathrm{H}$, deuterium${}_1{}^2\mathrm{H}$, and tritium ${}_1{}^3\mathrm{H}$.  ${}_1{}^4\mathrm{H}$does not exist.