The intermolecular space between molecules refers to the distance between the particles that make up a substance. When a substance is cooled, the kinetic energy of its particles decreases. This reduction in kinetic energy leads to changes in the intermolecular space. Let's explore this in more detail:

1. Explanation of intermolecular space:
- Intermolecular space refers to the space between molecules or particles in a substance.
- It is a result of the attractive forces between the particles, such as van der Waals forces, dipole-dipole interactions, or hydrogen bonding.
- The intermolecular space determines the physical properties of a substance, such as density, boiling point, and compressibility.

2. Effect of cooling on intermolecular space:
- Cooling a substance reduces the kinetic energy of its particles.
- As the particles lose kinetic energy, they slow down and move closer together.
- The attractive forces between the particles become more dominant as they approach each other.
- Consequently, the intermolecular space between molecules decreases.

3. Explanation of the correct option:
- The correct answer, option 'A' (It decreases), is supported by the explanation above.
- Cooling a substance reduces the intermolecular space because the particles move closer together due to decreased kinetic energy.
- This reduction in intermolecular space is observed in various substances when cooled, including gases, liquids, and solids.

4. Examples:
- In the case of a gas, cooling it can cause the particles to condense into a liquid or solid phase. This phase change is accompanied by a decrease in intermolecular space.
- For a liquid, cooling can lead to the formation of a solid as the particles arrange themselves in a more ordered manner, resulting in a decrease in intermolecular space.
- In a solid, cooling can cause the particles to vibrate less, leading to a decrease in intermolecular space between the fixed positions of the particles.

In summary, when a substance is cooled, the intermolecular space between molecules decreases. This is due to the reduction in kinetic energy of the particles, which causes them to move closer together and strengthen the attractive forces between them.