22) D

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.