Which of the following has the highest dipole moment?
The correct option is A Here, option (c) is already aromatic compounds in its non-polar state, so it has no dipole moment.
In ionic form of option (d), it has positive charge in the cyclopropenyl ring and it has 4π electron which obeys 4n rule. Hence its a antiaromatic compound. Since antiaromatic compounds are unstable the ionic form won't exist, hence it will be non-polar.
On, comparing option (a) and (b) compound, both forms ionic form but (b) has 2π electrons in the ring which obeys the (4n+2) Huckel's rule. Hence it is a aromatic compound. Since aromaticity gives more stability to the ionic form of compound (b), it will have more dipole moment than (a).