A four-pole alternator must produce 60 Hz output. Using the formula f = (poles × rpm) / 120, at what speed must the prime mover turn?

Why D is correct

Rearranging f = (poles × rpm) / 120: rpm = (f × 120) / poles = (60 × 120) / 4 = 1,800 rpm. A 2-pole machine would require 3,600 rpm and a 6-pole machine 1,200 rpm, making those plausible distractors. Per NEETS Mod. 5 §3-1, the governor must hold this speed precisely to maintain frequency.