To solve this problem, we will use the Doppler effect formula for sound.$\\$The formula for the observed frequency $f'$ when both the source and observer are moving is:$\\ f' = f \left( \frac{v + v_o}{v + v_s} \right) \\$where:$\\$• $f$ is the frequency of the source ($1392$ Hz).$\\$• $v$ is the speed of sound ($343$ m/s).$\\$• $v_o$ is the speed of the observer (motorcyclist) towards the source.$\\$• $v_s$ is the speed of the source (car) moving away from the observer.$\\$Convert the speeds from km/h to m/s:$\\$• Speed of the motorcyclist: $36$ km/h$= \frac{36 \times 1000}{3600} = 10$ m/s.$\\$• Speed of the car: $18$ km/h$= \frac{18 \times 1000}{3600} = 5$ m/s.$\\$Since the motorcyclist is moving towards the car, $v_o = 10$ m/s.$\\$Since the car is moving away from the motorcyclist, $v_s = 5$ m/s.$\\$Substitute these values into the Doppler effect formula:$\\ f' = 1392 \left( \frac{343 + 10}{343 + 5} \right) \\$Simplify the expression:$\\ f' = 1392 \left( \frac{353}{348} \right) \\$Calculate the observed frequency:$\\ f' = 1392 \times 1.01437 \approx 1412$ Hz$\\$Therefore, the frequency of the horn as heard by the motorcyclist is $1412$ Hz.$\\$This corresponds to Option 3.