Final answer:
The densities of stars in the Milky Way and Andromeda Galaxies are more representative of larger galactic structures within the universe and therefore closer to the overall density of stars in the universe, compared to the sparser Canis Major Dwarf Galaxy.
Step-by-step explanation:
The question asks which of the listed galaxies is closest in density of stars to the entire universe. If we analyze the given information, we can note specifics that help us determine the answer. The closest large galaxy to the Milky Way is the Andromeda Galaxy, which is very similar to our Milky Way. Both contain billions of stars and are bound by the same gravitational forces that influence movement within galaxies. The Canis Major Dwarf Galaxy, however, has only about 1% the mass of the Milky Way and is already merging with it, having its stars and clusters stripped and wrapped around the Milky Way.
Considering that the Milky Way and Andromeda are similar large spiral galaxies with vast numbers of stars, and that the Canis Major Dwarf Galaxy is much smaller and losing its stellar content to the Milky Way, it suggests that the densities of stars in the Milky Way and Andromeda are more representative of larger galactic structures within the universe as opposed to a dwarf galaxy like the Canis Major Dwarf Galaxy. Therefore, the Andromeda Galaxy (b) and the Milky Way (a) are more likely to have star densities closest to that of the entire universe, especially because dwarf galaxies tend to have lower star densities due to their smaller sizes and masses.
Given the similarity in the structure and population of stars between the Milky Way and Andromeda as compared to the relatively sparse Canis Major Dwarf Galaxy, the answer is either (a) the Milky Way or (b) Andromeda, with a slight lean towards the Andromeda due to the wording of the question that may suggest looking for a galaxy other than our own.