1 – Slow
H-H and I-I bonds are broken. Molecules H₂ and I₂ split to form 2HI. Since bonds are broken, this will be a relatively slow reaction.
2 – Fast
No bonds are broken. Ag+ and Cl- just react to form AgCl.
3 – Fast
Again, 2 molecules of CH₃ just bond together to form C₂H₆
4 – Slow
The C-Br bond is broken meaning this will be a relatively slow reaction.
Reaction 2 and 3 are the fastest so D is the answer.