Which of These Provides a Plausible Method to Measure the Mass

Which of these provides a plausible method to measure the mass of an exoplanet?

A) We use Newton's law of gravity, using the measured distance of the planet from its star and the planet's gravitational pull on the star.
B) We cannot make any firm estimate of the mass of an extrasolar planet with present technology.
C) We use spectra to measure the planet's temperature and photometry to measure its brightness.
D) We measure the planet's angular diameter and hence its size and then use spectra to find its composition and hence density.