The correct option is
C III
In ionic form of compound (I), both rings are aromatic in nature with 6 and 2
π-electrons. Hence, the ionic form will be more stable and have more dipole moment.
In ionic form of compound (II), again both rings are aromatic in nature with 6
π- electrons. Hence, the ionic form will be more stable and have more dipole moment.
In ionic form of compound (III), one ring is aromatic with 6
π- electron and other ring is antiaromatic with 4
π-electrons. This will destabilise the ionic form of the compound. Hence, its dipole moment will be comparitively lower than (I) and (II).