c
Here, speed of motorcyclist, $v_{m}=36 \mathrm{km}$ hour $^{-1}$
$=36 \times \frac{5}{18}=10 \mathrm{ms}^{-1}$
Speed of car,
$v_{c}=18 \mathrm{km} \text { hour }^{-1}=18 \times \frac{5}{18} \mathrm{ms}^{-1}=5 \mathrm{ms}^{-1}$
Frequency of source, $v_{0}=1392 \mathrm{Hz}$
Speed of sound, $v=343 \mathrm{ms}^{-1}$
The frequency of the honk heard by the motorcyclist is
$v^{\prime} =v_{0}\left(\frac{v+v_{m}}{v+v_{c}}\right)=1392\left(\frac{343+10}{343+5}\right)$
$=\frac{1392 \times 353}{348}=1412 \mathrm{Hz}$
