Chloroform is more acidic than fluoroform due to

CF₃ is less stable than CCl₃ due to the vacant d-orbital on the chlorine atom

The answer is (4) CF₃ is less stable than CCl₃ due to the vacant d-orbital on the chlorine atom.

The acidity of a compound depends on the stability of the conjugate base. The more stable the conjugate base, the more acidic the compound.

In the case of chloroform and fluoroform, the conjugate bases are CCl₃^- and CF₃^-, respectively. CCl₃^- is more stable than CF₃^- because the vacant d-orbital on chlorine can stabilize the negative charge.

Therefore, chloroform is more acidic than fluoroform.

The other options are incorrect.

Option (1) the electronegativity of fluorine is more than chlorine, is true, but it does not explain why chloroform is more acidic than fluoroform.

Option (2) the size of the chlorine atom is more than that fluorine atom, is also true, but it does not explain why chloroform is more acidic than fluoroform.

Option (3) fluorine is a second-period element having only s and p-subshells whereas chlorine is a third-period element having s, p, and d-subshells in which the d-subshell is vacant, is not relevant to the acidity of the compounds.