What is a planetary nebula? a. leftover gas from a supernova explosion b. the ejected envelope of a giant star surrounding the remnant of a star c. a type of young, medium-mass star d. a planet surrounded by a glowing shell of gas e. the disk of gas and dust surrounding a young star that will soon form a star system

b. the ejected envelope of a giant star surrounding the remnant of a star

Explanation:

A planetary nebula is the last stage of a Sun-like star. When a star dies it forms new celestial object. It can form either of planetary nebula, black hole, neutron star etc. depending on its mass.

When a Sun like star has run out of hydrogen to burn in the core it will expand to become a red giant and with time it will shed its outer layers in the form of a giant bubble or ring leaving a hot core in the center. This core will then contract to become a white dwarf star. The stellar winds further pushes this shell outwards and make it shine to present a beautiful and graceful celestial wonder that can be seen with optical telescopes.