When a plunger is pushed in, the air inside it gets compressed. This means that the volume of the air decreases. Let's understand why the correct answer is option 'A' - "No definite volume".

What is a plunger?
A plunger is a tool that is used to create suction or pressure to remove blockages from pipes or drains. It consists of a handle and a rubber cup or disc at the end.

Understanding the properties of air
Air is a gas and has certain properties that are different from solids and liquids. Two important properties of air are:

1. Definite shape: Air does not have a definite shape of its own. It takes the shape of the container it is in. For example, if air is in a balloon, it takes the shape of the balloon. If it is in a bottle, it takes the shape of the bottle.

2. No definite volume: Air does not have a fixed volume. It can be compressed or expanded depending on the external force applied to it. This means that the volume of air can change.

Compression of air in a plunger
When a plunger is pushed in, the air inside it gets compressed. This happens because the rubber cup or disc at the end of the plunger seals the opening, preventing the air from escaping. As the plunger is pushed, the volume of the air decreases because the air particles are forced closer together.

Why option 'A' is the correct answer?
The correct answer is option 'A' - "No definite volume". This is because when the plunger is pushed in, the volume of air inside it decreases. The air particles get compressed and occupy less space. This means that the air does not have a fixed or definite volume.

Conclusion
When a plunger is pushed in, the air inside it gets compressed, resulting in a decrease in volume. As a result, the correct answer is option 'A' - "No definite volume".