How do astronomers measure the masses of stars?

Generally the masses of stars are not measured directly. Masses are inferred from a star's spectral type and luminosity. The luminosity of a star pretty much depends on its mass, its age, and its chemical composition. This can be calculated using a computer model of the star. The luminosity can be measured, and the age can be estimated from the spectrum. The mass is then derived from these observed quantities.

The Sun is the one star where we know the mass and almost everything else about it accurately. It is used to calibrate and verify the computer models.

Masses can be calculated from the orbits of binary stars if the distance to the stars is known accurately and the orbital parameters derived from spectroscopic measurements of velocity or actual measurements of orbital position.

Lots of ways, the star's spectra can reveal a great deal about it's temperature which we can then determine a minimum mass, also with Kepler's "123" law, in other words, by studying the motions of objects in orbit around the star, you can determine it's mass.

i imagine it os C, because the Hertzsprung-Russel diagram is the in elementary words ay astronomers can somewhat degree the mass...i imagine that is C, if not C the probobly A... yet definately not B or D wish this enables