Which one of the following is not a planet of the sun?

Answer: (a) Sirius

Sirius is a star — the brightest in the night sky.

The idea

Membership of the solar system depends on being part of the Sun’s gravitational system. A bright object in the night sky can instead be a distant star with its own energy source. Being visible from Earth does not make an object a planet orbiting the Sun.

Also know

Sirius lies in the constellation Canis Major and is a binary system with a white-dwarf companion. Venus is a very bright planet and can appear as the morning or evening star, although it is not a star. A constellation is an apparent pattern on the sky; its stars need not be close together in space.