Correct option (c) 254 Hz
Explanation:
Let v be frequency of the unknown source. As it gives 4 beats per second when sounded with a
source of frequency 250 Hz,
Second harmonic of this unknown source = 492 Hz or 508 Hz which gives 5 beats per second. when sounded with a source of frequency 5 13 Hz.
Therefore unknown frequency, v - 254 Hz.