Answer:
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.
