Consider an infinite solenoid with a rectangular cross section instead of a circular cross section.
-(a) The solenoid's magnetic field must everywhere still point parallel to the solenoid's central axis.

Question

Quizplus · Accepted Answer

The shape of the cross-section does not affect the direction of the magnetic field inside a solenoid; it is always parallel to the solenoid's central axis due to the symmetry and the way the magnetic field lines are created by the current in the coils.

Consider an Infinite Solenoid with a Rectangular Cross Section Instead