What is the trend in the atomic radii of noble gases as we move down the group?

Which statements among the following are correct about Helium?
$(i)$ Liquid helium is used to sustain powerful superconducting magnets.
$(ii)$ Liquid helium is useful to carry low temperature experiments.
$(iii)$ It is a heavy gas.
$(iv)$ It is a flammable gas.

Which one of the following noble gases is the least polarizable?

In clathrates of $Xe$ with water,what is the type of bonding between $Xe$ and water molecules?

Assertion $(A)$: All noble gases are monoatomic.
Reason $(R)$: All noble gases have very low melting and boiling points.

Which one has the highest boiling point?