Black holes are
a. new stars
b. young stars
c. medium-aged stars
d. old stars (< i think this one.)

black holes are stars(usually) or other objects that have gained enough mass for their specific size to become a black hole its a mass density ratio, but in this case it should be old stars, since i forget the specifics of how and why but old stars seem to grow in size and mass and when they go super nova they may turn into black holes. but theoretically its not impossible for a newborn star to become a black hole since it just needs the right conditions to be meet for the possible new solar system to come to an untimely end. (if you didn't understand my rambling it is possible for all stars of all ages to become black holes under the right circumstances but the answer should be D. old stars).