Answer:
d. The interior region of a long current-carrying solenoid.
Step-by-step explanation:
The magnetic field produced by a straight wire is uniform but the magnetic field lines are not dense to strengthen it.
In case of a current carrying loop of wire the magnetic field always remains in a plane perpendicular to the tangential direction of current so the direction of magnetic field changes with space and hence it is not uniform.
Similar is the case for the magnetic field lines to the exterior of a solenoid for which the magnetic field lines vary with position in space and become rare as we move away from the pole ends.
- But on the inner space of the coil the magnetic field lines are parallel and dense when they become parallel near the poles.