Question

a groups of planets orbiting a single star
b. vast collections of stars bound together by gravity
c. clouds of gas without stars
d. remnants of exploded stars only
question 6 (1 point)
what happens when a star about 15 times the mass of our sun collapses?
a. it turns into a planet
b. it forms a stellar mass black hole
c. it becomes a comet
d. it explodes into an asteroid field

Explanation:

For the first question, galaxies are vast collections of stars bound together by gravity. Planets orbiting a single star form a planetary - system, clouds of gas without stars are not galaxies, and remnants of exploded stars are not what define a galaxy.
For the second question, when a star about 15 times the mass of our Sun collapses, it forms a stellar - mass black hole. A star does not turn into a planet, a comet, or explode into an asteroid field upon such a collapse.

Answer:

Question 1: b. vast collections of stars bound together by gravity
Question 2: b. it forms a stellar mass black hole