Due to presence of a triple bond between the two N-atoms, the bond dissociation enthalpy (941.4 kJ mol^-1) is very high.  Hence, N₂ is least reactive.

A compound which is most basic is obviously least acidic.  Since NH₃ is the most basic compound, therefore, it is least acidic.

A compound which is most acidic is obviously least basic.  Since SbH₃ is most acidic, therefore, it is least basic. or As the size of the atom increases, electron density decreases and hence basicity decreases.  Thus SbH₃ is least basic.

As we move down the group 15 elements from Sb to Bi, the atomic size increase and hence the strength of the bond it makes with hydrogen decreases, i.e., Bi-H bond is weaker than Sb-H bond.  In other words, BiH₃ is thermally less stable than sbH₃.  Therefore, on heating, BiH₃ has a greater tendency to liberate hydrogen than SbH₃ is a stronger reducing agent than SbH₃.

(i) NCl₅ (due to absence of d-orbitals)    
(ii) BiCl₅ (Bi⁵⁺ being an oxidising agent oxidises Cl^- ion to Cl₂ and Cl^-ion being a reducing agent reduces Bi⁵⁺ to Bi ³⁺).