What Is the Transit Method of Finding Extrasolar Planets

What is the transit method of finding extrasolar planets?

A) measuring the spectrum of the star for a periodic variation in its Doppler shift
B) monitoring a star to detect periodic dips in its brightness from the planet passing behind the star
C) observing a star during the rare occasion of another star passing directly in front of it; slight changes in how the light is bent by gravity can indicate the presence of planets around the closer star
D) monitoring a star to detect periodic dips in its brightness from the planet passing in front of the star