The transformer which is smallest in size and how to calculate that ? The options are
 100 KVA, 50 Hz
 100 KVA, 100 Hz
 100 KVA, 25 Hz
 50KVA, 25 HZ
Answer : 100 KVA, 100 Hz
Explanation

 The transformer equation
E = 4.44(f)(B)(N)(a)
 The transformer equation
Where,

 E is the voltage in the winding either primary or secondary,
 f is the frequency,
 B is the flux density in the core,
 N are the number of turns on the winding, and
 a is the cross area of the core.
 At higher frequencies the number of winding turns N is reduced.
 This formula follows from the Faraday formula, E = N dθ/dt and θ = B(a).
 This formula is switching power supplies use a much higher frequency (thousands of hertz) than 60 hertz, they use smaller transformers that don’t need as much wire as transformers operating at 60 hertz.