At the centre of which of the following four circular rings has maximum magnetic field while passing equal current through each one?

The direction of the induced magnetic field from a current carrying conductor is given by the Right hand thumb rule. In a case of a current carrying circular ring, magnetic field at the centre of the ring is coming out of the paper and perpendicular to the paper.

Now it is obvious that the magnetic field is stronger near the wire but when we come away from the wire it weakens and at the centre of the ring it is weakest. Hence, more the radius the ring the more magnetic field is weak at centre of the ring.

So, (d) will have the weakest magnetic field at centre and (a) has the strongest magnetic field at centre.