State of Matter:

There are four main states of matter: solid, liquid, gas, and plasma. Each state is determined by the arrangement of particles and the energy within the substance. Let's explore which of the following options is a state of matter.

Options:

- Cardboard: Cardboard is a material made from paper pulp, which is a solid material. It is not a state of matter but rather a substance that falls under the solid state of matter.

- Plasma: Plasma is a state of matter that consists of charged particles. It is often referred to as the fourth state of matter, alongside solid, liquid, and gas. Plasma is commonly found in stars, lightning, and neon lights.

- Oxygen: Oxygen is a gas at room temperature and pressure. It is one of the elements that can exist in multiple states of matter, depending on the conditions. In this case, oxygen is in the gaseous state.

- Liquid: Liquid is one of the main states of matter, characterized by a definite volume but no definite shape. Liquids can flow and take the shape of their container. Examples of liquids include water, milk, and oil.

Explanation:

Among the options provided, only plasma is a state of matter. Plasma is a unique state that exhibits properties different from solids, liquids, and gases. It is essential in various natural phenomena and technological applications. The other options, cardboard, oxygen, and liquid, are not states of matter but rather materials or substances that fall under specific states of matter.