Final answer:
A dwarf star is a star that is significantly smaller than giants or supergiants and includes main-sequence stars like red dwarfs and white dwarfs. Brown dwarfs, which exist between planets and stars in terms of size and mass, are also categorized as dwarf stars but are not true stars as they cannot sustain hydrogen fusion.
Step-by-step explanation:
A dwarf star refers to option D) a star that is significantly smaller than a giant or supergiant star, including main-sequence stars like red dwarfs and white dwarfs. Red dwarfs, for example, can have a diameter just 1/10 that of the Sun and masses around 1/12 that of the Sun, with an average density much higher than any known solid on Earth. Conversely, brown dwarfs are objects that fall in the intermediary category between planets and stars, with masses insufficient for regular hydrogen fusion but capable of deuterium fusion, and they have a size similar to that of Jupiter regardless of mass. The smallest true star—capable of sustaining hydrogen-into-helium fusion—has a mass of approximately 1/12 that of the Sun, while brown dwarfs range from roughly 13 to 80 Jupiter masses.