I would say the answer is A - a stars mass
The majority of stars in the galaxy, including our Sun, Sirius and Alpha Centauri A and B are all main sequence stars.
Mass is the key factor in determining the lifespan of a main sequence star, its size and its luminosity ( brightness)
Two characteristics define brightness: luminosity and magnitude. Luminosity is the amount of light that a star radiates. The size of the star and its surface temperature determine its luminosity. Apparent magnitude of a star is its perceived brightness, factoring in size and distance, while absolute magnitude is its true brightness irrespective of its distance from earth.
