Which characteristic is used to classify galaxies? the size of the galaxy the shape of the galaxy the presence of dust and gas between stars the presence of active star formation sites

the shape of the galaxy

The commonly used classification strategy for galaxies is based on one that was developed by Edwin P. Hubble and improved by an astronomer named Gerard de Vaucouleurs. It utilizes the three major kinds, and then much more divided by precise characteristics namely, the openness of spirals, size and extent of bars,and the size of galactic bulges. The sub-classifications also  include the age spectrum of its stars and the markers for such characteristics as a galaxy’s star-formation ratio in this age of multi-wavelength observing.

i think it is b) because they breathe and if you have seen an animal you would know that they do this by respiration.