Because the fixed point is far from the loop, we can approximate the current loop is an ideal magnetic dipole. The magnetic field due to an ideal magnetic dipole with dipole moment is given by
Therefore, if your location relative to the ideal magnetic dipole is held constant, then is proportional to the magnetic dipole moment . The dipole moment of a current loop is given by
where I is the current in the loop and a is the area of the loop. The area a for a circular loop of radius b is equal to , so the magnetic dipole moment equals , thus the magnetic field is proportional to . Hence answer (B) is correct.