Final answer:
The high speed of stars near the Milky Way's central black hole suggests that it has a large mass, specifically around 4 million solar masses, which significantly affects their orbital velocity and the dynamics of the galaxy.
Step-by-step explanation:
Given the high speed of stars near the black hole at the center of the Milky Way, we can infer that the black hole has a very large mass. The mass of the black hole has been estimated at about 4 million solar masses, based on the orbital dynamics of nearby stars. These stars move at significant velocities, which is a direct consequence of the black hole's gravitational force. According to the observations from radio telescopes and the application of Kepler's third law, all of this mass is confined within a region smaller than the orbit of Mercury, which signifies that the black hole is both incredibly dense and massive.
This concentration of mass creates a gravitational pull that influences the stars and other matter in its vicinity, leading to high orbital speeds. Such a supermassive black hole holds galaxies like our Milky Way together and plays a fundamental role in their dynamics. In this case, we can conclude that the correct answer is B. Its size, meaning the mass of the black hole, since 'size' in this context refers to mass rather than physical dimensions like volume or diameter.