Correct option is A
Slack bus:
In load flow studies, it is assumed that the losses in the transmission network can be supplied by one of the generators instead of shared by all the generators so that it is easy to calculate the loss.
The generator which supplies the entire transmission line losses and also shares the demand is called the slack bus. The remaining generators are only sharing the demand.
Power for slack bus = Total power going into the system - Total p going out of the system + transmission line losses.
This difference is called slack, and hence the name 'Slack Bus'.
When the system is having a greater number of buses the calculations can begin at one of the buses. The bus where the calculations are started is called as a Reference bus.
To make possible variations in real and reactive powers of the slack bus during the iterative process to be of a small percentage of its generating capacity, the bus connected to the largest generating station is normally selected as the slack bus.
Therefore, slack bus has to be a generator bus.
