Which element below is least reactive? helium oxygen fluorine nitrogen

The least reactive out of those options is helium. Alkali metals are very reactive while noble gases are stable, and helium is a noble gas.

The least reactive is the helium.

Explanation:

Reactivity is defined as the ability of an element to combine with others. The least reactive element is helium, because it belongs to the group of noble or inert gases. These gases aren't very reactive, because their outermost layer or valence layer is always complete. This causes the element to be so stable that it tends not to react with others.

Oxygen, nitrogen and fluorine belong to the group of nonmetals. Nonmetals have a tendency to resemble the closest noble gases in terms of the electronic configuration of their last layer, having a tendency to accept electrons. Then they are reactive.

Fluorine only needs one more electron to be stable, oxygen needed two and nitrogen three. So the reactivity of the fluorine is greater (besides it is an atom so small that its electrons are very close to the nucleus), then the oxygen follows and finally the nitrogen.