Final answer:
Sirius B is less luminous but hotter than the Sun, classifying it as a white dwarf. Since it formed at the same time as Sirius A yet has already become a white dwarf, its initial mass was greater, allowing it to evolve more quickly.
Step-by-step explanation:
Sirius B is hotter than the Sun but much less luminous; it is an example of a white dwarf. Sirius B, being a white dwarf, cannot have a mass greater than 1.4 Msun, which is known as the Chandrasekhar limit. Sirius A is still on the main sequence with a mass of about 2 Msun. Considering both stars formed at the same time, the progenitor of Sirius B must have been more massive than Sirius A to have evolved faster and become a white dwarf before Sirius A left the main sequence.
Images taken by the Hubble Space Telescope and the Chandra X-Ray Telescope show the differences in visible and X-ray light between Sirius A and B. Despite being fainter in visible light, Sirius B outshines Sirius A in X-rays due to its higher temperature. This illustrates the contrasting properties between different types of stars, such as red giants and main-sequence stars.
Answer: D) white dwarf