Final answer:
The hottest type of stars is the Spectral type-O main-sequence star. Therefore, the correct option is D.
Step-by-step explanation:
Stellar temperatures are categorized using the spectral classification system, where O-type stars are the hottest, followed by B, A, F, G, K, and M in decreasing order of temperature. Therefore, the correct answer among the options provided would typically be A, as O-type stars are not included.
These spectral classes correspond to the surface temperature of stars, with O-type stars having the highest temperatures, emitting a blue-white light. It's important to note that the absence of O-type in the options may lead to a potential oversight, and A-type stars are generally hotter than those in lower spectral classes.
Therefore, the correct option is D.