Inert gases have very high ionization enthalpy and zero electron gain enthalpy. So the correct answer is He.

Option E is the correct answer.

Group 18 contains noble gases.

Xe is present in 5th period of periodic table.

Helium is an inert gas. Inert gases are not very reactive as their outermost shell is completely filled. $ {He} $ is not very soluble in water.

As per octet rule, noble gases have 8 electrons in their outermost shell. According to octet rule the element which has 8 electron in its valence shell are stable.

Hydration decreases with a decrement in charge density of an ion

Lithium is lightest elemental metal nearly half as dense as water, so it floats on the surface and ultimately disappears, giving off hydrogen gas and forming a colourless solution of lithium hydroxide.

Sodium reacts with water more vigorously than lithium its more electropositive and will give electrons easily.
Hence option C is correct.

Sodium ions and potassium ions do not react with water.
The main reason is that have already lost that electron and are therefore much more stable. They don't tend to lose another electron, and they certainly don't try to get that lost electron back, so there's no tendency for them to react.

Answer (A)

If we add sodium to water it floats on the water and reacts very vigorously, rushing around and on the surface producing hydrogen gas and sodium hydroxide. Hence to reduce its vigorousness, it is amalgamated with mercury by making sodium as amalgamating sodium. 

Lithium and sodium form complex hydrides such as lithium aluminium hydride and sodium borohydride.