Noble gases do not react with other elements because
A) They have completely paired up and stable electron shells
B) The sizes of their atoms are very small
C) Are not found in abundance
D) Are monoatomic

Hint: Noble gases are the elements of the periodic table's 18^th group. They are helium, neon, argon, xenon, etc. They are known as non-reactive elements. Here, we have to identify the correct reason for their non-reactive nature.

Complete Step by Step Solution:
Let's discuss option A first. It says that noble gases are fully paired and have stable electron shells. This is correct because as we see from Helium, Neon, Argon, Xenon, etc. they all possess fully paired valence shells. Therefore, they attain stability and are thus non-reactive in nature.

Option B says the atomic sizes of noble gases are very small. It is not correct because size increases from helium to radon because of the increase of shells. Therefore, option B is not the reason for the non-reactive nature of the noble gases.

Option C says noble gases are found in abundance. It is wrong because only argon is found in abundance on Earth. Therefore, option C is also wrong.

Option D says helium gases are monoatomic. Noble gases are monoatomic because of their non-reactivity. But option D is the consequence of the non-reactivity of the noble gases. Therefore, D is also wrong.
Hence, option A is right.

Note: It is to be noted that the noble gases have no smell and colour. Their nature is inflammable and they are monoatomic gases that mean molecules of noble gases exist as one atom such as He, Ne, etc.