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(B) The frequency $f'$ of the sound received by the pipe from a source moving towards it with speed $u$ is given by the Doppler effect formula: $f' =

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$A, D$

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(B) The frequency $f'$ of the sound received by the pipe from a source moving towards it with speed $u$ is given by the Doppler effect formula: $f' = f_s \left( \frac{v}{v - u} \right)$.$A$ closed pipe of length $L$ resonates at odd harmonics of its fundamental frequency $f_0$,i.e.,$f' = n f_0$,where $n = 1, 3, 5, 7, \dots$.$(A)$ For $u = 0.8 v$ and $f_s = f_0$: $f' = f_0 \left( \frac{v}{v - 0.8 v} \right) = f_0 \left( \frac{v}{0.2 v} \right) = 5 f_0$. Since $5$ is an odd integer,this leads to resonance.$(B)$ For $u = 0.8 v$ and $f_s = 2 f_0$: $f' = 2 f_0 \left( \frac{v}{v - 0.8 v} \right) = 10 f_0$. Since $10$ is an even integer,this does not lead to resonance.$(C)$ For $u = 0.8 v$ and $f_s = 0.5 f_0$: $f' = 0.5 f_0 \left( \frac{v}{v - 0.8 v} \right) = 2.5 f_0$. This is not an odd harmonic.$(D)$ For $u = 0.5 v$ and $f_s = 1.5 f_0$: $f' = 1.5 f_0 \left( \frac{v}{v - 0.5 v} \right) = 1.5 f_0 \left( \frac{v}{0.5 v} \right) = 3 f_0$. Since $3$ is an odd integer,this leads to resonance.Thus,combinations $(A)$ and $(D)$ lead to resonance.

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