Which of the following is an example of the doppler effect

"A train blows its horn as it travels towards you, and you perceive the pitch of the horn to increase" is an example of the Doppler effect.

Explanation:

Christian Doppler, physicist from Austria was the first to launch "Doppler effect" in 1842. It is variation in wave frequency in the response to a body or object which move nearby the waves sources. The phenomenon is observed in sound, light and other waves.

As the source of the wave moves closer to the listener the frequency of the wave increases in sound waves while the sound starts to faint when the source moves away.

Doppler effect is applicable in a mechanism called red shift or blue change in order to calculate the velocity at which stars and galaxies approach or recede from us.