The magnetic field inside a long straight solenoid carrying current:
(a) is zero
(b) decreases as we move towards its end.
(c) increases as we move towards its end.
(d) is the same at all points.

The magnetic field inside a long straight solenoid carrying current is same at all points because magnetic field lines are parallel to each other hence form a uniform field strength. However, it reduces at an end when the lines move apart.