Which alkyl halide has highest bond enthalpy of $\mathrm{C}-\mathrm{X}$ bond?

Calculate van't Hoff factor (i) of 0.2 m aqueous solution of an electrolyte if it freezes at $-0.660 \mathrm{~K} .\left(\mathrm{K}_{\mathrm{f}}=1.84 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}\right)$

Identify reagent used for preparation of benzophenone from benzonitrile?

Which of the following is an elementary reaction?