Which of the following is a star?
(A) Sun
(B) Mercury
(C) Pluto
(D) Venus

Hint: A planet is anybody which orbits a star and has cleared its orbit of the bodies. A planet can be anything from a small rock, body to a large Gas giant.

Complete step by step solution:
The sun is the star at the center of the solar system. It is a nearly perfect sphere of hot plasma heated to incandescence by nuclear fusion reactions in its care radiating the energy mainly as light and infrared radiation. It is the most important source of energy for life on Earth. While Mercury, Pluto and Venus are the planets which move in fixed orbit around the sun.

Note: When a high-mass star has no hydrogen left to, burn it expands and becomes a red supergiant. While most start quickly face away, the Supergiant’s destroy themselves in a huge explosion, called a supernova.