Final answer:
The correct statement about stars is that star formation begins in a nebula. There are more white dwarfs than planetary nebulae because white dwarfs have a much longer visibility lifespan compared to the short-lived planetary nebulae.
Step-by-step explanation:
The correct statement about stars is A. Star formation begins in a nebula. This is a process where clouds of gas and dust, known as nebulae, collapse under gravity to form new stars. As for the remaining statements, B is incorrect because white dwarfs do not become main-sequence stars again even if they gain mass. Instead, if a white dwarf accumulates enough mass from a companion star, it might explode as a Type Ia supernova. C is incorrect because supergiants cannot absorb black holes; in fact, supergiants may eventually collapse to form black holes themselves if they are massive enough. Finally, D is incorrect as main-sequence stars are not formed by comets; instead, they form from the gravitational collapse of gas and dust within a nebula.
There are more white dwarfs than planetary nebulae in the Galaxy because the planetary nebula phase is relatively short-lived compared to the lifespan of a white dwarf. After the planetary nebula is ejected, the white dwarf remains and can be seen for billions of years as it slowly cools. On the other hand, the gas of the planetary nebula disperses into space and becomes less visible over time.