C-halogen bond length totally depends on the electronegativity difference between Carbon and halogen. The more the difference and the less is the bond length. The electronegativity order of halogens is I<Br<Cl<F. Hence, the correct order of increasing C-halogen bond length is CH3F<CH3Cl<CH3Br<CH3I.
Or we can say that as the size of halogen increases C-halogen bond length also increases.