The correct option is B O<F<Cl
In general, the electron gain enthalpy becomes more negative on going from left to right in a period while it becomes less negative on going from top to bottom in a group.
But F has a less electron gain enthalpy than Cl because in F, on adding another electron externally to its valence shell, it causes severe electron-electron repulsion due to which the system becomes comparatively unstable. Whereas, adding an electron to Cl is quiet convenient because of the larger size which is due to the large size of the 3rd shell.
Whereas, O = 1s22s22p4 has a less negative value than F because, on going from left to right in the period, nuclear charge increases.
Therefore, the correct order of increasing electron gain enthalpy is: O<F<Cl