Dipole moment is highest for
 

As the number of chlorine atoms increases symmetrical nature increases
$\mathrm{μ}$ of CH₃Cl > CH₂Cl₂ > CHCl₃ > CCl₄

Alternate explanation...CCl₄ is a symmetrical molecule with zero dipole moment. CH₃Cl,CH₂Cl₂,CHCl₃ have distorted tetrahedral shape .As the number of 'Cl' atoms increases repulsions between bond pairs increases thus bond angle (θ) increases ,dipole moment decreases.