Answer:

To avoid segregation, fresh concrete should be dropped from a height of less than one meter. This is the correct answer because dropping concrete from a height greater than one meter can lead to segregation, which is the separation of coarse aggregate from the cement paste. Segregation can negatively affect the quality and strength of the hardened concrete.

Reasoning:

1. Segregation in Concrete:
Segregation in concrete occurs when the ingredients of the mixture separate, resulting in non-uniform distribution and a non-homogeneous mixture. This can happen if the concrete is not properly mixed or if it is handled improperly during placement.

2. Causes of Segregation:
Segregation can be caused by various factors, including:
- Dropping concrete from a height: When fresh concrete is dropped from a certain height, the impact can cause the heavier coarse aggregates to settle at the bottom, while the lighter cement paste rises to the top. This leads to an uneven distribution of the aggregates and the cement paste, resulting in segregation.
- Excessive vibration: Vibrating concrete excessively can also cause segregation by causing the aggregates to separate from the cement paste.
- Improper mix design: If the proportions of the ingredients in the concrete mix are not properly balanced, it can result in segregation.

3. Impact of Dropping Concrete from a Height:
When fresh concrete is dropped from a height, the force of impact can cause the heavier coarse aggregates to settle at the bottom, while the lighter cement paste rises to the top. This separation of the aggregates and the cement paste leads to an uneven distribution, resulting in segregation. The higher the drop height, the greater the impact force, and the more severe the segregation.

4. Optimal Drop Height:
To minimize segregation, it is recommended to drop fresh concrete from a height of less than one meter. This height is considered optimal because the impact force is relatively low, allowing for a more even distribution of the aggregates and the cement paste. Dropping concrete from a height less than one meter helps to prevent the heavier aggregates from settling at the bottom and the lighter cement paste from rising to the top, resulting in a more homogeneous mixture.

Conclusion:
To avoid segregation in fresh concrete, it is best to drop it from a height of less than one meter. This ensures a more uniform distribution of the aggregates and the cement paste, leading to a higher quality and stronger hardened concrete. Proper handling and placement techniques are crucial in achieving a homogeneous mixture and preventing segregation in concrete.