Fluorine is the most reactive among all the halogens, because of it's:

The correct option is D low dissociation energy of F - F bond

Fluorine is the most reactive among all the halogens, because of it's low dissociation energy of $F-F$ bond. The dissociation energies (kcal/mol) in $F_2,C{l}_2,B{r}_2$ and $I_2$ are 38, 57, 45.5 and 35.6 respectively.

In the case of fluorine:

(a) short bond length and larger repulsion between non-bonding electrons lead to low dissociation energy.

(b) absence of multiple bonding leads to low dissociation energy.