The most useful noble gas is:
A.\[He\]
B.\[Ar\]
C.\[Ne\]
D.\[Xe\]

Hint:We should be familiar with the properties of all the noble gases. It is necessary to have a brief idea about the occurrence of these inert gases because some of them are prepared in laboratories with certain set of conditions.

Complete step by step answer:
Group 18 of the periodic table contains gases named; Helium\[\left( {He} \right)\], Neon\[\left( {Ne} \right)\], Argon\[\left( {Ar} \right)\], Krypton\[\left( {Kr} \right)\], Xenon\[\left( {Xe} \right)\] and Radon\[\left( {Rn} \right)\]. Because of the completely filled valence shell, members of this family don’t like to form compounds. That is why; this group is called noble gases or inert gases.
These gases have stable electronic configuration, so these gases are chemically inert, have high ionization energy and positive electron gain enthalpy.
 Let us discuss the uses of these gases.
\[He\] is used to fill hot air balloons, in oxygen cylinders by scuba divers, used as coolant in superconducting magnets. Liquid \[He\] is used as cryopreservative. \[He\] is abundant in the universe(hydrogen is the most abundant gas) but is not present in the Earth’s atmosphere, thus it is produced from natural gas wells (for commercial purpose) and other possible means . Because of this, \[He\] is the most useful noble gas.
\[Ne\] is used in lamps.
\[Ar\] is used to provide an inert atmosphere and is present in the Earth’s atmosphere.
\[Xe\] and \[Kr\] are used in making special colored bulbs. Fluorides of\[Xe\]are known to us. We also study Xenon –Oxygen compounds. For Krypton, \[Kr{F_2}\]is known.
\[Rn\] is a radioactive element obtained from a radioactive decay of Radium. Compounds of\[Rn\] are known, but we have not achieved the milestone in isolating them yet.

Thus, option A. is the correct answer for the given question.

Note:
Noble gases except \[He\] and\[Rn\] are present in the earth’s atmosphere, thus they are obtained using fractional distillation and liquefaction, whereas\[He\] is produced from natural gas wells and\[Rn\] is produced by the radioactive decay of Radium.