Introduction:
Brake power is an important parameter that determines the performance and efficiency of a diesel engine. It refers to the power output of an engine measured at the engine's crankshaft. In order to increase the brake power of a diesel engine while keeping other parameters constant, one needs to manipulate the intake air.

Explanation:
The intake air plays a crucial role in the combustion process of a diesel engine. It provides the necessary oxygen for combustion and influences the overall efficiency of the engine. By manipulating the intake air, we can optimize the combustion process and increase the brake power of the engine.

Decreasing the Density of Intake Air:
If we decrease the density of the intake air, there will be fewer oxygen molecules available for combustion. This will result in incomplete combustion and a decrease in brake power. Therefore, decreasing the density of intake air is not a suitable option for increasing brake power.

Increasing the Temperature of Intake Air:
Increasing the temperature of the intake air can lead to a more efficient combustion process. When the air is heated, its density decreases, allowing more air to enter the combustion chamber. This results in better mixing of fuel and air, leading to improved combustion and increased brake power. Therefore, increasing the temperature of the intake air is a suitable option for increasing brake power.

Increasing the Pressure of Intake Air:
Increasing the pressure of the intake air can also enhance the combustion process. When the air is compressed, its density increases, allowing more oxygen to be packed into the combustion chamber. This leads to better combustion and increased brake power. Therefore, increasing the pressure of intake air is a suitable option for increasing brake power.

Decreasing the Pressure of Intake Air:
Decreasing the pressure of the intake air will result in a decrease in the amount of oxygen available for combustion. This will lead to incomplete combustion and a decrease in brake power. Therefore, decreasing the pressure of intake air is not a suitable option for increasing brake power.

Conclusion:
To increase the brake power of a diesel engine while keeping other parameters constant, the pressure of intake air needs to be increased. This allows for better combustion and improved efficiency, resulting in an increase in brake power.