The phrasing of your question makes me wonder what you are actually asking.

An alternator voltage regulator works very differently from a traditional linear regulator. An automotive alternator voltage regulator nominally works by modulating the current applied to the rotor coil so that a constant output voltage exists on the other side of the alternator diode bridge, the same node as the battery. This way as both the engine's available mechanical energy and electric load changes, a constant voltage gets maintained at the battery.

Is it a question in your test paper ?

Did you go through your text books? it is available (and in our times at least) there was almost half chapter on it.

Regulation is:

{(Full Load Volatge - No Load Voltage) / (No load Voltage)} x 100.

It is not the error of the alternator but the internal voltage drop in the alternator windings which is maximum at full load.