The correct answer is D. All of the above.
Air cooling is a method of cooling an object by circulating air around it. This can be done by using fans, blowers, or other devices to move the air. Air cooling is often used in turbo-alternators because it is a relatively simple and efficient way to remove heat from the rotor.
One sided axial ventilation is a type of air cooling that uses a fan to blow air across the rotor from one side. This type of ventilation is relatively simple and inexpensive, but it does not provide as much cooling as other methods.
Two sided axial ventilation is a type of air cooling that uses two fans to blow air across the rotor from both sides. This type of ventilation provides more cooling than one sided axial ventilation, but it is also more expensive.
Multiple inlet system is a type of air cooling that uses multiple fans to blow air across the rotor from multiple directions. This type of ventilation provides the most cooling of all the methods, but it is also the most expensive.
In conclusion, all of the methods mentioned in the question can be used for air cooling of turbo-alternators. The best method to use depends on the specific application and the desired level of cooling.