c
(c)

Time \(= t\;s\)

Distance \(= xm\)

\(y=A \cos 240(t-x / 12)\)

Distance \(=0.5 m\)

\(y = A \cos 240\left(\frac{240 t-240 x }{12}\right)\)

To find the wavelength,

\(k =240 / 12\)

\(k =20\)

\(k\) is Wave number.

\(\lambda=2 \pi / 20\)

\(\lambda\) is the wavelength.

\(0.5 / \lambda=\theta / 2 \pi\)

\(0.5 /(2 \pi / 20)=\theta / 2 \pi\)

\(\theta=10^0\)

The phase difference between two positions \(0.5 m\) apart is \(10^{\circ}\)