prob 2

a)
use electro negativity concept

electronegativity order here is Cl>Br>I .....par usske aage kya??

The more electronegative the halogen, the more strongly will the electrons be attracted. So the bond energy will be more.

You can also reason it out based on the max overlap theory
In C-X bond, the 2p orbital of the C bonds with the 3p for Cl, 4p for Br and 5p for I while forming the bond

As the bond is more stable if there is max overlap and this is possible if the energy levels of the bonding orbitals is nearly same,

So the overlap in case of 2p-3p > 2p-4p > 2p-5p

So C-Cl > C-Br > C-I