Final answer:
On average, the type of galaxy with the least mass is an irregular galaxy, which typically has a lower mass than both spiral and elliptical galaxies, even though dwarf ellipticals are also less massive within the elliptical category.
Step-by-step explanation:
The type of galaxy with the LEAST mass is typically an irregular galaxy. While elliptical galaxies can include both the most massive and least massive varieties, irregular galaxies have lower average masses compared to spirals. Elliptycal galaxies show a wide range from giant elliptical to dwarf ellipticals; however, the giants far exceed spirals in mass.
Dwarf ellipticals, which can be challenging to observe due to their faint appearance, possess the lesser masses among ellipticals but are still more massive than typical irregular galaxies. Irregular galaxies are characterized by their disorganized structure and are often sites of intense star formation activity. Even though these galaxies may contain both young and old stars, their overall masses are typically lower than those of spiral galaxies.