Answer:
The correct answer is D. The majority of visible stars are main sequence stars.
Explanation:
The Main Sequence in a Hertzsprung-Russell diagram is a curve along which most stars are. A star in the main series is called a Main Sequence star.
The sun is an example of a main series star: it has been there for about 4.5 billion years and will continue so for another about 4.5 billion years. After the hydrogen in the core of the sun has been consumed, it will begin to burn helium, swell and become a red giant star.
