Final answer:
The most massive main-sequence stars on the Hertzsprung-Russell (HR) diagram are located in the upper-left corner. They are the hottest and most luminous stars, typically of spectral type O, and much more luminous than our Sun.
Step-by-step explanation:
On a standard Hertzsprung-Russell (HR) diagram, the most massive main-sequence stars are found at the upper-left position (the correct answer is D. Upper-Left). The HR diagram plots stars according to their absolute magnitude (brightness) against their spectral class (temperature). As we look at the diagram, the least massive stars are located at the lower right, being both cool and dim. Moving diagonally across the diagram to the upper left, stars increase in both temperature and luminosity.
The upper-left corner of the HR diagram is where you would find the most massive and luminous main-sequence stars. These stars are of spectral type O, exceedingly hot, and emit a blue light indicating very high temperatures. They stand out on the HR diagram not only because of their massive size but also due to their extraordinary luminosity, often exceeding a million times the luminosity of our Sun. Thus, the answer to the student's question is D. Upper-Left.