Which of the following orders is correct with respect to the magnitude of enthalpies of formation for alkali metal halides?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R).$
Assertion $(A):$ Lithium salts are hydrated.
Reason $(R):$ Lithium has higher polarising power than other alkali metal group members.
In the light of the above statements,choose the most appropriate answer from the options given below:

When potassium is heated strongly in oxygen,it forms:

Due to low ionization energy,alkali metals are...

In the presence of which of the following does magnesium continue to burn?

The atomic radii of alkali metals $(M)$ lie in the order $Li < Na < K < Rb$,but the radii of $M^{+}$ ions in aqueous solution lie in the reverse order $Li^{+} > Na^{+} > K^{+} > Rb^{+}$. What is the reason for this reverse order (on going from $Li$ to $Rb$)?