Answer: rate of joule heating
Explanation: When an iron rod is inserted inside a solenoid the magnetic field inside the solenoid increases due to the high magnetic permeability of iron. Due to the increased magnetic field, the magnetic flux also increases. The self-inductance of the solenoid also increases because the self-inductance (L) of the coil is directly proportional to the permeability of the material inside the solenoid. The rate of joule heating doesn’t change.