(a) Metallic radii increase in a group from top to bottom. Thus, Li < Na < K < Rb is true. (b) Electron gain enthalpy of Cl > F and afterwards it decreases along a group. (c) Ionisation enthalpy increase along a period left to right but due to presence of half-filled orbital in N, ionisation enthalpy of N > O. Thus B < C < N < O is incorrect. (d) For isoelectronic species, ionic size ∝