Concept:Inside a long, current-carrying solenoid, the magnetic field is uniform and the field lines are straight and parallel to the axis.
Explanation:A solenoid is a cylindrical coil made of many closely spaced turns of insulated copper wire.
When an electric current passes through it, a magnetic field is generated that closely resembles the field of a bar magnet.
One end of the solenoid behaves as the north pole and the other as the south pole.
For a solenoid that is very long compared to its diameter, the magnetic field inside is nearly uniform.
The field lines inside are straight lines parallel to the axis of the solenoid.
This implies that the magnetic field strength is the same at every point inside the solenoid.
Outside the solenoid, the field lines curve and spread out, similar to a bar magnet.
The field pattern inside can be understood by considering the superposition of the fields from each turn.
For an ideal infinite solenoid, the field inside is perfectly uniform and parallel.
Therefore, among the given options, the field lines are in the form of parallel straight lines.
Answer:Parallel straight lines.
