Which of the following elements does not have a neutrons in its nucleus

The element hydrogen has total three isotopes. They are proton, deuterium and tritium. Let us see the fundamental particles present in each one.

${}_1{}^1\mathrm{H}$(proton) →1 proton+ no neutron

 ${}_1{}^2\mathrm{H}$(Deuterium) → 1 proton + 1 neutron 

${}_1{}^3\mathrm{H}$(Tritirum) → 1 proton + 2 neutrons 

${}_2{}^4\mathrm{He}$(Helium) → 2 proton +2 neutrons