Which of the following is classified as a birthplace of stars?

The correct classification for a birthplace of stars is a nebula. Nebulae are vast clouds of gas and dust in space where the conditions are suitable for the formation of new stars. The process begins when parts of the nebula collapse under their own gravity, leading to increased density and temperature, ultimately forming protostars. As the protostar continues to gather mass from the surrounding material, it may eventually ignite nuclear fusion and become a main-sequence star.

Nebulae can serve as the essential environments where new stars are born, making them foundational to stellar evolution. Though protostars are indeed stages in star formation, they are not classified as the birthplace itself but rather as the transitional phase that occurs after initial formation in a nebula. Therefore, the nebula is the correct answer since it refers to the broader environment where star birth occurs.