Final answer:
Supergiants are the stars with the highest temperatures on the Hertzsprung-Russell diagram, located in the upper left corner, characterized by their very high surface temperatures and luminosity.
Step-by-step explanation:
According to the Hertzsprung-Russell diagram, the stars with the highest temperatures are typically those found in the upper left corner of the diagram. These are the stars known as supergiants. The H-R diagram plots stellar luminosity against surface temperature, and supergiants are characterized by their high luminosity and very high surface temperatures. In fact, these supergiants are the most massive and luminous of the main-sequence stars and often exceed 100 times the mass of the Sun. They are very hot and blue, belonging to the spectral type O. Therefore, the correct answer to the student's question is (d) Supergiant.
Main-sequence stars form the bulk of the stars on the H-R diagram, making up about 90% and extending diagonally from high temperature and high luminosity to low temperature and low luminosity. White dwarfs, on the other hand, are found at the lower left corner of the H-R diagram, indicating that they are both hot and dim, but not as hot as the supergiants. Giants fall somewhere in between but are characterized as being cool and bright. All of these star types represent different stages in a star's life cycle.