a
(a) Since source of frequency $x$ gives $8$ beats per second with frequency $250 Hz, $
it's possible frequency are $258$ or $242.$
As source of frequency $x$ gives $12$ beats per second with a frequency $270 Hz,$
it's possible frequencies $282$ or $258 Hz.$
The only possible frequency of $x$ which gives $8$ beats with frequency $250 Hz$ also $12$ beats per second with $258 Hz.$