The correct answer is option 4.

Option 1 states that the magnetic field inside the solenoid is uniform, which is correct. A solenoid is a coil of wire that produces a magnetic field when an electric current passes through it. The magnetic field inside a solenoid is indeed uniform because the magnetic field lines are parallel and evenly spaced along the length of the solenoid.

Option 2 states that a current-carrying solenoid behaves like a bar magnet, which is also correct. When a current passes through a solenoid, it creates a magnetic field similar to that of a bar magnet. One end of the solenoid behaves like a north pole, while the other end behaves like a south pole.

Option 3 states that the magnetic field inside the solenoid increases with an increase in current, which is correct. The strength of the magnetic field produced by a solenoid is directly proportional to the current flowing through it. Increasing the current passing through the solenoid will result in a stronger magnetic field inside the solenoid.

Option 4, however, is incorrect. If a soft iron bar is inserted inside the solenoid, the magnetic field inside the solenoid does not remain the same. Soft