Which type of star is characterized as little, hot, and dim, being the leftover center of an old star?

A white dwarf is the remnant core of a star that has exhausted the nuclear fuel for fusion reactions. After a star similar in size to our Sun has completed its life cycle, it shed its outer layers, often forming a planetary nebula, while the core collapses and becomes increasingly dense. In this stage, the white dwarf is typically small—about the size of Earth—but extremely hot and packed with the remaining mass of the star. Despite its small size, a white dwarf is incredibly dense and can shine brightly due to the residual heat it retains, although it does not produce new energy through fusion. Over time, it will cool and dim further, eventually fading away.

The other options refer to different aspects of stellar evolution or types of stars. A supernova is the explosive end of a massive star, a neutron star is the incredibly dense remnant left after a supernova for certain massive stars, and a protostar is an early stage in the formation of a star when it is still gathering mass from its surroundings. Therefore, the characteristics of being little, hot, and dim make the white dwarf the correct answer.