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

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

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

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

The wavelength of the sound in the region in front of the moving car is the apparent wavelength due to the source moving towards the observer. It is given by:

$\lambda' = \dfrac{v - v_s}{f}$

Substituting the given values:

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

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

$\lambda' = 0.8 \text{ m} = 80 \text{ cm}$