Two point charges  +q and -q  are held fixed at (-d,0) and (d,0) respectively of a (X,Y) coordinate system. Then 

From figure, it is clear that Ê  at all points on the y-axis is along Î . Here Ê of all points on x-axis cannot have the same direction.
Here electric potential at origin is zero so no work is done in bringing a test charge from infinity to origin.

Here dipole moment is in – x direction (– q to +q).
Hence only option (a) is correct.