c
(c) The tuning fork whose frequency is being tested produces $2$ beats with oscillator at $514 Hz,$ therefore, frequency of tuning fork may either be $512$ or $516.$
With oscillator frequency $510$ it gives $6$ beats/sec, therefore frequency of tuning fork may be either $516$ or $504.$
Therefore, the actual frequency is $516 Hz$ which gives $2$ beats/sec with $514 Hz$ and $6 $ beats/sec with $510 Hz.$