Which is the best leaving group? a) I- b) Br- c) Cl- d) F

Which is the best leaving group? a) I- b) Br- c) Cl- d) F

Explanation

 Leaving group ability:

A good leaving group is a species that can easily depart from a molecule taking a pair of electrons with it. 

Halide trend:

Among halogens (I, Br, Cl, F), iodide (I-) is the best-leaving group because it is the weakest base, meaning it is most stable with a negative charge.