The correct answer is B. Ninth
Step-by-step explanation:
Since its discovery in the 20th century, Pluto was believed to be the ninth planet of our solar system as this is the ninth most massive object that orbits the sun, and it is located after Neptune which is the eighth planet in our solar system. However, in 2006 Pluto was classified as a dwarf planet because different from other planets such as Earth, Mars, Venus, Saturn, etc. Pluto was not massive enough to clear its orbit, and due to this, it could not be classified as a regular planet. Thus, in the past and until 2006 Pluto was considered the ninth planet.