As we move down the halogen group in the periodic table, the size of the halogen atom increases from fluorine to iodine. Consequently, the carbon-halogen ($\text{C-X}$) bond length increases from $\text{C-F}$ to $\text{C-I}$ . Since bond enthalpy is inversely proportional to bond length, the bond enthalpy decreases as the bond length increases. According to standard data, the bond enthalpies are: $\text{CH}_3\text{-F}$ (452 kJ/mol), $\text{CH}_3\text{-Cl}$ (351 kJ/mol), $\text{CH}_3\text{-Br}$ (293 kJ/mol), and $\text{CH}_3\text{-I}$ (234 kJ/mol) . Thus, the correct decreasing order of bond enthalpy is $\text{CH}_3\text{-F} > \text{CH}_3\text{-Cl} > \text{CH}_3\text{-Br} > \text{CH}_3\text{-I}$.