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

\(v^{\prime} =v_{0}\left(\frac{v+v_{m}}{v+v_{c}}\right)=1392\left(\frac{343+10}{343+5}\right)\)

\(=\frac{1392 \times 353}{348}=1412 \mathrm{Hz}\)