My understanding is that the output voltage is a function of field strength and rotational speed. If the rotational speed is the same then the task of the voltage regulator must be to keep the field strength constant to achieve a constant voltage output.
In theory field windings can be in series or in parallel with the loads. In this context 'loads' are things you can switch on, they are always arranged in parallel in the circuit so the more loads you switch on the lower the overall circuit resistance and the greater the current that flows (I = V ÷ R)
If the field windings are in series with the loads it means that, as you switch more things on, the current also increases through the field windings and output voltage increases. Not good.
If the field windings are in parallel with the loads then, as you switch more things on, there is a greater tendency for electrons to flow through the loads and less tendency for them to flow through the field coils, the current in the field coils drops and output voltage reduces. Not good.
If you carefully put some of the field windings in series and some in parallel you can arrange it so that output voltage stays roughly constant when you switch things on.
The key word here is 'roughly'. To get more accurate voltage control you tweak it so there is a slight tendency for output voltage to fall as loads increase, in order to do this you put slightly more of the field windings in parallel.
Now we have a situation where, as loads increase, the excitation current in the field coils falls slightly, this reduces output voltage slightly. The job of the voltage regulator is to adjust (reduce) the resistance of the parallel circuit with the field coils in it so that current flow through the coils is restored to its previous level and output voltage restored.
Thus the full sequence should be:
Loads increase, excitation current falls, voltage output falls, voltage regulator increases field current, voltage increases to previous level.
.....and you could answer this (a) or (b) depending on which stage of the process you thought the examiner was asking about. Acting on 'information received' we believe the 'proper' answer is (b).
Any AEOs or electrical engineers out there do pitch in and correct me if I've got this wrong.