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 $Rb$)?

• A
  Gradual increase in ionisation energy
• B
  Increasing weakness of the metallic bond
• C
  Increasing electropositive character
• D
  Decreasing degree of hydration