Assertion: A beaker filled with water at 4oC overflows if the temperature is decreased or increased.

Reason: Density of water is maximum at 4oC.

Explanation:
The assertion states that a beaker filled with water at 4oC will overflow if the temperature is either decreased or increased. The reason provided is that the density of water is maximum at 4oC.

Understanding the Assertion:
When water is heated, its molecules gain kinetic energy and move faster, leading to an increase in the space between them. As a result, the volume of water expands, causing it to overflow if the beaker is not large enough to accommodate the increased volume.

When water is cooled, the molecules lose kinetic energy and move slower. This causes the molecules to come closer together, reducing the space between them. As a result, the volume of water contracts, and if the beaker is not large enough to accommodate the decreased volume, it will overflow.

Since the assertion states that the beaker overflows both when the temperature is decreased and increased, it implies that the volume change is significant in both scenarios.

Understanding the Reason:
The reason provided states that the density of water is maximum at 4oC. This is a well-known property of water, and it can be explained by the unique behavior of water molecules.

At temperatures below 4oC, water molecules form a lattice-like structure due to hydrogen bonding. This structure takes up more space, causing the volume of water to increase. As a result, the density decreases.

When the temperature is increased from 4oC, the hydrogen bonds break, and the water molecules gain kinetic energy, leading to an increase in the space between them and a subsequent increase in volume. The density decreases further as a result.

However, at 4oC, the hydrogen bonding is at its maximum, and the water molecules are packed closely together, resulting in the highest density. Any increase or decrease in temperature from this point will cause the volume to change, leading to a decrease in density.

Explanation of the Correct Answer:
The correct answer is option 'A': Both A and R are true, and R is the correct explanation of A.

The reason provided in the explanation adequately explains the assertion. The change in density of water with temperature is responsible for the overflow of the beaker when the temperature is either increased or decreased. The maximum density of water at 4oC is the key factor that leads to significant volume changes and subsequently causes the beaker to overflow.