At what speed, a 4-pole alternator has to be driven, so as to generate a voltage at a frequency of 25 Hz? frequency of 25 Hz?

Correct option is C

​$$\text{The synchronous speed of an alternator is given by:} \\[6pt]N_s = \frac{120 f}{P} \\[10pt]\text{where} \\[6pt]\bullet \ N_s = \text{ synchronous speed (rpm)} \\[4pt]\bullet \ f = \text{ frequency (Hz)} \\[4pt]\bullet \ P = \text{ number of poles} \\[12pt]\textbf{Given:} \\[6pt]\bullet \ f = 25 \text{ Hz} \\[4pt]\bullet \ P = 4 \\[12pt]\textbf{Calculation:} \\[8pt]N_s = \frac{120 \times 25}{4} = \frac{3000}{4} = 750 \text{ rpm}$$​​