c
(c)

\(v=350 \,m / s \quad f=500 \,Hz\)

\(v=\frac{\omega}{k}\)

\(k=\frac{2 \pi f}{v}\)

\(k=\frac{2 \pi \times 500}{350}\)

\(k=\frac{2 \pi}{0.7}\)

\(\therefore \lambda=0.7 \,m\)

Let equation of wave be

\(y=A \sin (k x-\omega t)\)

Let \(x_1=0\) \(x_2=1 \,m\)

\(y_1=A \sin (\omega t)\)

\(y_2=A \sin \left(\frac{2 \pi}{0.7} \times 1+\omega t\right)\)

Hence phase difference \(=\frac{20 \pi}{7}\) or approximately

Phase difference \(\approx 3 \pi\)

Which is same as \(\Delta \phi=\pi\)