What are the three main states of matter?

The three main states of matter are solid, liquid, and gas. Solids have a defined shape and volume, as the particles are closely packed together in a fixed arrangement. Liquids have a definite volume but take the shape of their container because the particles are less tightly packed and can move past one another. Gases, on the other hand, have neither a definite shape nor a defined volume, as the particles are far apart and move freely, filling the entire space available to them.

Other options introduce terms like plasma, which is indeed a state of matter but is less commonly referenced than the traditional three. Vapor and gel are also not classified as main states but rather describe specific physical forms or transitions of matter. Thus, the correct answer emphasizes the foundational understanding of matter's states crucial for further studies in chemistry and physics.