If the TPM of an axial-flow compressor remains constant, how can the angle of attack on the rotor blades be changed?

The correct choice is based on understanding how the relationship between flow velocity and blade angles affects the angle of attack in an axial-flow compressor. The angle of attack is defined as the angle between the relative wind (the airflow moving over the blades) and the chord line of the blade.

When the total pressure maintained by the compressor (referred to here as the TPM, or total pressure margin) is constant, one effective way to change the angle of attack on the rotor blades is by altering the velocity of the airflow. As the velocity of the airflow increases, the relative wind changes, which can increase or decrease the angle of attack on the rotor blades without needing to physically adjust the position of the blades themselves.

If the airflow speed is reduced, the angle of attack increases, whereas increasing the airflow speed decreases the angle of attack. This principle is important in maintaining aerodynamic efficiency and preventing stalls in the compressor.

Adjusting the angle of the stator blades, altering the fuel flow, or changing the temperature of the air directly affects other parameters like pressure and temperature but does not inherently change the airflow velocity, thus not directly affecting the angle of attack as described in this context.