The correct answer is option 'D': the alternator will continue to run as a synchronous motor running in the same direction.

Explanation:

1. Introduction to Alternator and Infinite Bus:
An alternator is a machine that converts mechanical energy into electrical energy. It operates on the principle of electromagnetic induction. An infinite bus refers to a system with a large amount of power available and is capable of supplying or absorbing any amount of power without changing its voltage and frequency.

2. Prime Mover Shutdown:
The prime mover of an alternator refers to the mechanical source that drives the alternator, such as a turbine or a diesel engine. If the prime mover is suddenly shut down, it means that the mechanical power to the alternator is cut off.

3. Synchronous Operation:
In normal operation, an alternator functions as a generator, converting mechanical power into electrical power. However, when the prime mover is shut down, the alternator is no longer driven by mechanical power. As a result, the alternator cannot generate electrical power on its own.

4. Synchronous Motor Operation:
When the prime mover is shut down, the alternator can still continue to operate, but now as a synchronous motor. A synchronous motor operates by synchronizing its speed with the frequency of the electrical system to which it is connected. In this case, the alternator will synchronize with the frequency of the infinite bus.

5. Same Direction Operation:
The question states that the alternator will continue to run as a synchronous motor running in the same direction. This means that the direction of rotation of the alternator will be maintained even after the prime mover shutdown.

6. Synchronization and Power Flow:
Once the alternator operates as a synchronous motor and synchronizes with the infinite bus, it will start to draw electrical power from the infinite bus. The power flow will be in the same direction as the original generator operation.

7. Stability Considerations:
It is important to note that the stability of the system should be carefully monitored when the prime mover is shut down. Sudden disconnection of a large load or the loss of a prime mover can lead to instability in the electrical system. Protective relays and control systems are typically used to monitor and stabilize the system in such situations.

In summary, when the prime mover of an alternator supplying load to an infinite bus is suddenly shut down, the alternator will continue to operate as a synchronous motor running in the same direction. It will synchronize with the infinite bus and draw power from it.