The answer is: A. Self-excited generator
A self-excited generator is a type of generator that uses its own output voltage to excite the field winding. This means that the generator can maintain a constant output voltage even as the load changes.
A separately excited generator is a type of generator that uses an external source of power to excite the field winding. This means that the generator cannot maintain a constant output voltage if the load changes.
A level compounded generator is a type of generator that has a field winding that is designed to produce a constant output voltage regardless of the load. However, level compounded generators are not as common as self-excited generators.
In conclusion, a self-excited generator is the only type of generator that can offer constant voltage on all loads.