Which element from following exhibits highest number of various different possible oxidation states?

Which among the following has lowest boiling point?

A buffer solution is prepared by mixing 0.01 M HCN and 0.02 MNaCN . If $\mathrm{K}_{\mathrm{a}}$ for HCN is $6.6 \times 10^{-10}$, what is the concentration of $\mathrm{H}^{+}$ions in solution?

Which reagent from following is used for preparation of aliphatic aldehyde from nitriles?