Answer: d) The bulging center of a spiral galaxy consisting of stars in the shape of a flattened sphere that circles a black hole
Explanation:
Bulge is part of star at the center where stars are closely packed. Most of the spiral galaxies have Bulge at their heart that is at their center. The shape of a bulge resembles that of a flattened sphere. Most of the bulges have high compact mass at their center about which the stars rotate. This high compact mass at the center is assumed to be super-massive black-hole.
Thus, a nuclear bulge can be defines as: d) The bulging center of a spiral galaxy consisting of stars in the shape of a flattened sphere that circles a black hole