c
(c) Critical hearing frequency for a person is \(20,000Hz\).

If a closed pipe vibration in \({N^{th}}\) mode then frequency of vibration

\(n = \frac{{(2N - 1)v}}{{4l}} = (2N - 1){n_1}\)                 (where \({n_1} = \) fundamental frequency of vibration)

Hence \(20,000   = (2N - 1) \times 1500\) ==> \(N = 7.1 \approx 7\)

Also, in closed pipe Number of over tones = (No. of mode of vibration \(-1\)) 

\(= 7 -1 = 6.\)