Which element has the least protons in the nucleus oxygen, lithium, hydrogen, Carbon?

The one with the least protons is hydrogen

The element has the least protons in the nucleus is Hydrogen.

Explanation:

The three main things that forms an atom are proton, neutron and electron. The negatively charged are electrons and positively charged are protons. Neutrons do not contain any charges. The positive charge of one proton will be balanced by the negative charge of one electron.

The element that contains only one proton and one electron is hydrogen. The protons are present in the nucleus and the electrons will be revolving around it. The atomic number of hydrogen is 1. This because of the reason that there is only one proton and no neutron in it.