The correct sequence of bond enthalpy of ‘C–X’ bond is :

The correct option is: CH₃−F > CH₃−Cl > CH₃−Br > CH₃���I.

To understand the trend in bond enthalpy of the carbon-halogen ($\text{C}-\text{X}$) bond in methyl halides (${\text{CH}}_3\text{X}$), we need to look at the factors that affect bond strength:

1. Size of the Halogen Atom: As we move down Group 17 (from fluorine to iodine) in the periodic table, the atomic size of the halogens increases progressively:
$\text{F}<\text{Cl}<\text{Br}<\text{I}$

2. Orbital Overlap and Bond Length: Since the carbon atom uses its 2p orbital for bonding, the overlap is most effective with the small 2p orbital of fluorine. As the size of the halogen's valence orbital increases ($3\text{p}$ for Cl, $4\text{p}$ for Br, and $5\text{p}$ for I), the extent of overlap between the carbon sp³ orbital and the halogen p orbital decreases. Consequently, the $\text{C}-\text{X}$ bond length increases:
${\text{CH}}_3-\text{F}<{\text{CH}}_3-\text{Cl}<{\text{CH}}_3-\text{Br}<{\text{CH}}_3-\text{I}$

3. Bond Enthalpy: Longer bonds are weaker and require less energy to break. Therefore, the bond enthalpy (bond dissociation energy) decreases as the bond length increases. This gives us the following decreasing order of bond enthalpies:
${\text{CH}}_3-\text{F}>{\text{CH}}_3-\text{Cl}>{\text{CH}}_3-\text{Br}>{\text{CH}}_3-\text{I}$