What is the order of reactivity of alkyl halides with ammonia?

Reactivity of alkyl halides with ammonia depends on nucleophilic substitution mechanisms, where ammonia acts as a nucleophile. The key factor is the leaving group ability of the halide ion.

Leaving group ability is inversely related to basicity. Comparing halides: iodine is the weakest base and thus the best leaving group, followed by bromine and then chlorine. This gives the order I^- > Br^- > Cl^- for leaving group quality.

Consequently, alkyl halide reactivity follows: $\mathrm{R}{-}\mathrm{I} > \mathrm{R}{-}\mathrm{B} > \mathrm{R}{-}\mathrm{Cl}$.

Evaluating the options:
A) $\mathrm{R}{-}\mathrm{Cl} > \mathrm{R}{-}\mathrm{B} > \mathrm{R}{-}\mathrm{I}$ (Incorrect)
B) $\mathrm{R}{-}\mathrm{B} > \mathrm{R}{-}\mathrm{I} > \mathrm{R}{-}\mathrm{Cl}$ (Incorrect)
C) $\mathrm{R}{-}\mathrm{I} > \mathrm{R}{-}\mathrm{B} > \mathrm{R}{-}\mathrm{Cl}$ (Correct)
D) $\mathrm{R}{-}\mathrm{Cl} > \mathrm{R}{-}\mathrm{I} > \mathrm{R}{-}\mathrm{B}$ (Incorrect)

Answer: $\boxed{\text{C}}$