Final answer:
The most common type of stars are red dwarfs, corresponding to group d. They are smaller, cooler, and red in color, with a long lifespan due to their slow burning of hydrogen fuel.
Step-by-step explanation:
The most common type of stars in the universe are red dwarfs, which would correspond to group d in your question. These stars are smaller and cooler than our sun, and they make up about 75% of all the stars in the universe. Their red color comes from the fact that these stars are cooler than others, with temperatures around 3,500 degrees Kelvin. Their smaller size and lower temperature mean they burn their hydrogen fuel more slowly, allowing them to last for billions, and sometimes trillions, of years.
Learn more about Red Dwarfs