To detect an extrasolar planet by means of the doppler shift, you look for a periodic shift of the spectrum lines of the a. star the planet is orbiting. b. planet. c. star and the planet.

A. star the planet is orbiting

Explanation:

The doppler effect is in the form of electronic waves such as light waves used in astronomy also called as the red or blue shift.  Used to measure the speed at which the stars and galaxies orbital motion i.e approach or recede around us. This shift happens on a small scale. it is also used to detect an Exoplanet hiding in the space.