Final answer:
Helium requires the least amount of energy to remove an electron from the outermost energy level.
Step-by-step explanation:
The element that requires the least amount of energy to remove an electron from the outermost energy level is He (helium). Helium has only two electrons, with both electrons in the 1s orbital. The first ionization energy is the energy required to remove an electron from a gaseous atom, and for helium, this energy is very low because the 1s orbital is complete with only two electrons.