The net force experienced by a bar magnet placed in a uniform magnetic field is zero. This means that the forces acting on the magnet in different directions cancel each other out, resulting in a net force of zero.

Option 1: Zero - This option is correct. When a bar magnet is placed in a uniform magnetic field, the magnetic forces acting on the magnet are balanced, resulting in a net force of zero.

Option 2: Depends upon the length of the magnet - This option is incorrect. The net force experienced by a bar magnet in a uniform magnetic field does not depend on the length of the magnet. It is determined by the balance of forces acting on the magnet.

Option 3: Never zero - This option is incorrect. The net force experienced by a bar magnet placed in a uniform magnetic field is indeed zero.

Option 4: Depends upon temperature - This option is incorrect. The net force experienced by a bar magnet in a uniform magnetic field is not dependent on the temperature. The temperature does not affect the balance of forces acting on the magnet.

In conclusion, option 1 is the correct answer as the net force experienced by a bar magnet placed in a uniform magnetic field is zero.