Travelling towards a cliff, a car moves with a speed of 54 km h^ -1...

Speed of the car, $v_c = 54 \text{ km h}^{-1} = 54 \times \dfrac{5}{18} \text{ m s}^{-1} = 15 \text{ m s}^{-1}$

Frequency of the horn, $f_0 = 400 \text{ Hz}$

Speed of sound, $v = 335 \text{ m s}^{-1}$

The apparent frequency of the sound reaching the cliff is given by the Doppler effect for a moving source and stationary observer:

f' = f_0 \left(\dfrac{v}{v - v_c} \right)

The cliff acts as a stationary source reflecting the sound at this frequency $f'$. The wavelength of the reflected sound wave is:

$\lambda' =$\dfrac{v}{f'} = \dfrac{v - v_c}{f_0}

Substituting the given values:

$\lambda' =$\dfrac{335 - 15}{400}

$\lambda' =$\dfrac{320}{400} =$0.8 \text{ m}$

$\lambda' = 80 \text{ cm}$