Dipole moment (μ) is the product of distance between two poles and charge, which can be represented as- μ= e X d, where e= charge and d= separation between two poles.
Polarity in any bond arises due to electronegativity difference between two bonded atoms. C-Cl bond as well as C-F bond are polar in nature because Cl and F-atoms are electronegative and C-atom is electropositive. Dipole moment is a vector quantity, so it is dependent on direction also.
1-Chloro-2-fluoroethane is CH₂(Cl)CH₂(F). Here both C-atoms are sp³ hybridised and are attached be C-C single bond. So rotation of the C-C bond is possible. The molecule can exist in two forms- eclipsed and staggered form. In eclispsed conformation, both electronegative atoms Cl and F are in same direction whereas in staggered conformation Cl and F-atoms are in opposite direction.
When two bond moments are in opposite direction, net dipole moment of the bond will be zero and when two bond moments are in same direction, net dipole moment will not be zero. Therefore, eclipse conformation of 1-chloro-2-fluoroethane will have greatest dipole moment.