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 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.

the shape of the galaxy