Final answer:
The Sun is classified as a spectral type-G main-sequence star, which is in the middle of the classification scale that ranges from the hottest and most luminous O-type stars to the coolest and least luminous M-type stars.
Step-by-step explanation:
The main sequence stars include different spectral types, which classify stars based on their surface temperature and luminosity. Among these, the Sun is classified as a spectral type-G main-sequence star, specifically as a G2 star. The spectral classification scale goes in the order of O, B, A, F, G, K, M, with O being the most massive, hottest, and most luminous, and M being the least massive, coolest, and least luminous on the main-sequence. Therefore, for the question regarding which category the Sun belongs to among O-type, B-type, G-type, and M-type main-sequence stars, the correct answer is c) G-type.