Which of the following elements is inert?
A. Fe
B. He
C. Cl
D. Ag

Hint: An element is said to be inert if the valence shell of that element is completely filled or has a complete duplet or octet and belongs to the 18th group of the periodic table. The word ‘inert’ relates to the chemical inertness of the element.

Complete step by step answer:
Helium is an inert gas and a part of the 18th group elements also known as the inert gases. It has a completely filled valence shell and no extra electron which can easily react with any other elements or reacting species. The inert gases are highly stable elements and have a high value of ionization enthalpy and electron gain enthalpy. This means that the noble gases require a very high amount of energy to lose or gain an electron in order to form their respective cations or anions and to react with any neighboring species present in the system. The chemical reactivity of these elements is very low.
In the case of Helium, it is the second lightest gas and is odorless and colorless in nature. Helium follows a complete duplet structure in which it has 2 electrons in its valence shell and has a completely filled stable electronic configuration with low reactivity.

Thus, the correct option is B.

Note:
An inert element is the one which has the least tendency to react with the surrounding species. All the noble elements are inert in nature due to a completely filled valence shell electronic configuration.