Final answer:
The incorrect option is (b), 'The magnetic field of a solenoid is constant,' as it can vary with current and coil parameters, while option (a) can be correct under proper magnetic induction procedures.
Step-by-step explanation:
The question involves concepts from Physics, particularly magnetism and the behavior of magnetic fields in different contexts. Option (b), stating 'The magnetic field of a solenoid is constant,' is the incorrect option. While a solenoid can create a magnetic field when an electric current passes through it, the magnetic field's intensity can vary depending on factors like the current and the number of coils. If by 'constant' it is implied that the field doesn't change over the length of the solenoid, then it is somewhat correct for an ideal solenoid, but in practice, the field can vary especially near the ends of the solenoid.
Option (a), 'The bar magnet, with the help of another bar magnet, becomes a permanent magnet,' could be correct if the bar magnet is properly aligned and stroked with another magnet to align the domains in the same direction, as per magnetic induction, which can make it a permanent magnet.