Question

Magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid. Why?

Answer 1

Explanation:

the magnetic field lines always form closed loops. as the turns of wires in a toroidal solenoid are wounded over it's core in circular form; the field lines are confined within the core of the toroid.

In a straight solenoid; the magnetic field lines can't form close loops within a solenoid.