Which statement explains why elements in Group VIII of the Periodic Table are unreactive?
(A) They are monatomic gases
(B) They form stable diatomic molecules
(C) They have a full outer shell of electrons
(D) They share electrons with each other

Hint: Chemically inert is a word used in chemistry to describe a material that is not chemically reactive. A material is thermodynamically inert, or nonlabile, if it is thermodynamically unstable (positive standard Gibbs free energy of creation) yet decomposes slowly or not at all.

Complete answer:
Noble gases make up Group 18 of the modern periodic table. Noble gases include helium, neon, argon, krypton, xenon, and radon. These gases are called inert gases because they are monatomic and chemically inert under typical circumstances. These gases are known as rare gases because they are only found in trace amounts in the environment. Radon is a radioactive gas with radioactive properties. Because these gases were not found during Mendeleev's lifetime, he did not include them in the periodic table. To accommodate them in the contemporary periodic table, a new group known as zero groups was developed later. Group zero has been changed to group 18 in accordance with IUPAC norms.
Under normal circumstances, noble gases do not react. Noble gases are non-reactive due to the following factors:
The noble gases' valence shells are totally filled. The next accessible shell has a significantly greater energy level.
The ionisation enthalpies of noble gases are quite high.
They have high positive electron gain enthalpy values.
Hence option C is correct.

Note:
When elements react, their atoms lose, acquire, or share electrons to complete their outer shells. Noble gas atoms already have entire outer shells, therefore they aren't inclined to lose, gain, or share electrons. This is why noble gases are chemically inert and do not participate in reactions.