Final answer:
M-type stars are the most common main sequence stars due to their low mass, while O-type stars are the rarest and most massive, making them the most luminous and hottest.
Step-by-step explanation:
The most common type of main sequence stars are the M-type stars, while the O-type stars are the rarest. This is because stars are much more likely to be low-mass than high-mass.
The most massive main-sequence stars (spectral type O) are also the most luminous and have the highest surface temperature. Conversely, the lowest-mass stars on the main sequence (spectral type M) are the least luminous and the coolest. Therefore, the correct answer to the question is: Most common: M-type stars, Rarest: O-type stars.