This question was previously asked in

ISRO (VSSC) Technician (Electrician) Sept 2016 Official Paper

Option 3 : 750 r. p. m

__Concept:__

The synchronous speed of a synchronous machine is given by

\({N_s} = \frac{{120f}}{P}\)

Where f is frequency and P is the number of poles.

A synchronous machine always rotates at a synchronous speed.

__Calculation:__

Number of poles (P) = 8

Frequency (f) = 50 Hz

Synchronous speed, \({N_{s}} = \frac{{120 \times 50}}{8} = 750~rpm\)