Answer:
A. observing the planet's effect on its host star.
Step-by-step explanation:
It is very hard to detect the planets directly as they do not have luminosity of their own, so scientist have developed an alternate method to detect planets indirectly. This is done by observing the effects that the planet has on the star that it surrounds. One of the methods is Transit Photometry.
In this method we detect distance planets by measuring minute dimming a star as the orbiting planet crosses between it and earth.