Correct Answer - Option 2 : Number of poles and rotational speed
The rotor speed of the alternator is given by
\(N = {{120f} \over P}\) ........(1)
Where,
N = Speed of rotor in rpm
f = Frequency of generated emf
P = Number of Poles
From equation (1)
\(f = {{NP} \over 120}\)
\(f \propto {NP} \)
So the output frequency of an alternator depends on both the number of poles and rotational speed.