Answer:
Star:
- A star is a celestial object that generates its own light and energy through nuclear fusion in its core.
- Stars are composed primarily of hydrogen and helium.
- They exist in different sizes, colors, and temperatures.
- Stars can be categorized into different types such as main sequence stars, giant stars, dwarf stars, and supergiant stars.
- The Sun is considered a star.
- It is located at the center of our solar system.
- The Sun is classified as a main sequence star of spectral type G2V.
- It is approximately 4.6 billion years old and will continue to shine for several billion more years.
- The Sun's energy sustains life on Earth through the process of photosynthesis and provides heat and light.
Moon:
- The Moon is Earth's natural satellite.
- It does not generate its own light but reflects the sunlight.
- The Moon orbits around the Earth and takes about 27.3 days to complete one revolution.
- It has phases, such as full moon, new moon, crescent moon, and gibbous moon, depending on its position relative to the Earth and the Sun.
- The Moon has a rocky surface and lacks an atmosphere.
- It influences tides on Earth due to its gravitational pull.
Earth:
- Earth is the third planet from the Sun in our solar system.
- It is a terrestrial planet with a solid surface and an atmosphere.
- Earth is the only known planet to support life.
- It has a diverse range of ecosystems and is home to millions of species.
- Earth has a magnetic field that protects it from harmful solar radiation.
- It takes approximately 365.25 days to complete one revolution around the Sun.
Comet:
- A comet is a small celestial object composed of ice, dust, and rock.
- Comets have a highly elliptical orbit around the Sun.
- When a comet approaches the Sun, the heat causes the ice to vaporize, forming a glowing coma and often a tail.
- Comets are often visible from Earth during their close approach to the Sun.
Conclusion:
- Among the given options, the Sun is the star.
- The Moon is a satellite, Earth is a planet, and a comet is a different type of celestial object.

Explanation:

Star:
- A star is a celestial body that produces light and heat due to nuclear reactions happening in its core.
- The Sun is a star located at the center of our solar system.
- It is a massive ball of gas, primarily consisting of hydrogen and helium.

Moon, Earth, and Comet:
- The Moon is Earth's natural satellite, which reflects light from the Sun.
- Earth is a planet that orbits around the Sun, receiving light and heat from it.
- A comet is a small icy body that orbits the Sun and develops a glowing coma and tail due to the heat of the Sun.

Therefore, among the options given, the Sun (option A) is the only star, while the Moon, Earth, and comet are not stars but other celestial bodies in our solar system.