Explanation:

Dipole Moment:
Dipole moment is a measure of the separation of positive and negative charges in a molecule. It is a vector quantity and is represented by a symbol μ (mu). The dipole moment is directly proportional to the magnitude of the charge and the distance between the charges.

Effect of Electronegativity:
In a covalent bond, if the two atoms have different electronegativities, the electron density is shifted towards the more electronegative atom. As a result, a partial positive charge is developed on the less electronegative atom, while a partial negative charge is developed on the more electronegative atom. This separation of charges creates a dipole moment.

Effect of Molecular Geometry:
The dipole moment also depends on the molecular geometry of the molecule. If the molecule is symmetric, the dipole moments of individual bonds cancel out each other, resulting in a net dipole moment of zero. However, if the molecule is asymmetric, the dipole moments do not cancel out, resulting in a non-zero net dipole moment.

Analysis of Options:
a) CH3Cl: Chlorine (Cl) is more electronegative than carbon (C) and hydrogen (H). The C-Cl bond is polar due to the difference in electronegativity. The molecule has a pyramidal shape with the chlorine atom at the apex. The dipole moments of the C-H bonds do not cancel out due to the asymmetry of the molecule. Therefore, CH3Cl has a net dipole moment.
b) CH2Cl2: This molecule also has a tetrahedral shape with two chlorine atoms and two hydrogen atoms bonded to a central carbon atom. The dipole moments of the C-H bonds cancel out each other, resulting in a net dipole moment of zero.
c) CHCl3: This molecule has a trigonal pyramidal shape with three chlorine atoms and one hydrogen atom bonded to a central carbon atom. The dipole moments of the C-H bonds do not cancel out due to the asymmetry of the molecule. Therefore, CHCl3 has a net dipole moment.
d) CCl4: This molecule has a tetrahedral shape with four chlorine atoms bonded to a central carbon atom. The dipole moments of the C-Cl bonds cancel out each other, resulting in a net dipole moment of zero.

Conclusion:
Among the given options, CH3Cl has the highest dipole moment as it is the only molecule with an asymmetric molecular geometry that does not cancel out the dipole moments of the C-H bonds.