In Doppler effect only change in frequency is observed and not change in wavelength if observer approaches the source.
$I.$ The speed of the wave is $4n \times ab$
$II.$ The medium at $a$ will be in the same phase as $d$ after $\frac{4}{{3n}}s$
$III.$ The phase difference between $b$ and $e$ is $\frac{{3\pi }}{2}$
Which of these statements are correct
Statement$-2:$ Intensity of waves of given frequency in same medium is proportional to square of amplitude only.