Correct order of bond dissociation energy

Which have melting point below $$500^{\circ} \mathrm{C}$$ ?

Which has least covalent radius?

Which pair of elements has maximum electronegativity difference?

In which of the following arrangements, the order is not strictly according to the property written against it?

The ionic radii $$(\mathop A\limits^o)$$ of $$\mathrm{C}^{4-}$$ and $$\mathrm{O}^{2-}$$ respectively are 2.60 and 1.40. The ionic radius of the isoelectronic ion $$\mathrm{N}^{3-}$$ would be

Assertion (A) Electron gain enthalpy becomes less negative as we go down a group.

Reason (R) Size of the atom increases on going down the group and the added electron would be farther away from the nucleus.