The correct answer to the question "What is a meteor?" is Option 2: a piece of matter which has entered the earth`s atmosphere from outer space.

A meteor is a phenomenon that occurs when a small piece of solid matter, known as a meteoroid, enters the Earth`s atmosphere from outer space. As it travels through the atmosphere, the friction caused by the meteoroid`s high speed and the interaction with air molecules heats up the meteoroid, causing it to glow and produce a streak of light across the sky. This glowing streak is what we commonly refer to as a meteor or a shooting star.

Here is a detailed reasoning for why Option 2 is the correct answer:

1. Option 1: A rapidly moving star: This option is incorrect because a meteor is not a star. While meteors may appear to move rapidly across the sky, they are not stars. Stars are massive celestial objects that emit light and are located at vast distances from Earth, whereas meteors are small solid objects that burn up in the atmosphere.

2. Option 2: A piece of matter which has entered the Earth`s atmosphere from outer space: This option is correct. As mentioned earlier, meteors are formed by meteoroids, which are small rocky or metallic objects that originate from asteroids or comets. When a meteoroid enters the Earth`s atmosphere, it undergoes intense heating due to the air friction, resulting in the glowing streak we observe.

3. Option 3: Part of a constellation: This option is incorrect. While meteors can occur anywhere in the sky, they are not specifically associated with constellations. Constellations are patterns of stars that are visible from Earth and have been named and recognized by astronomers. Meteors can be observed in any part of the sky and are not limited to specific constellations.

4. Option 4: A comet without a tail: This option is incorrect. While comets and meteors are related phenomena, they are not the same thing. Comets are celestial bodies composed of ice, dust, and small rocky particles, and they often have a distinct glowing coma and a tail when they approach the Sun. Meteors, on the other hand, are the result of meteoroids entering the Earth`s atmosphere and burning up, causing a brief streak of light.

In summary, a meteor is a piece of matter (meteoroid) that enters the Earth`s atmosphere from outer space. It undergoes intense heating due to air friction, resulting in a glowing streak of light commonly known as a shooting star or meteor.