Answer:

Introduction:
In areas prone to flooding, it is crucial to design and construct highways in a way that allows for the passage of floodwater without causing damage to the road or hindering the flow of water. One of the most effective methods for achieving this is by building the highway on an embankment.

Embankment:
An embankment is an elevated mound of earth or rock that is constructed to raise the level of a road or railway track above the surrounding ground. It is typically built using compacted soil or other suitable materials to provide a stable foundation for the highway.

Advantages of an embankment:
- Elevation: By constructing the highway on an embankment, it is raised above the flood level, ensuring that floodwater can pass beneath it without causing any obstruction.
- Stability: The embankment provides a stable base for the highway, preventing it from being washed away or damaged during floods.
- Drainage: Embankments are designed with proper slope and drainage systems to ensure that water does not accumulate on the road surface, reducing the risk of hydroplaning.
- Accessibility: Unlike other options such as subways or underpasses, an embankment allows for continuous access to the highway during normal conditions, minimizing disruptions to traffic flow.
- Cost-effectiveness: Building an embankment is generally more cost-effective compared to constructing subways or underpasses, as it requires less excavation and construction materials.

Disadvantages of other options:
- Subway: A subway involves constructing a tunnel beneath the flood-prone area. However, building and maintaining a subway can be expensive and may require complex engineering solutions to ensure water does not enter the tunnel during floods.
- Overpass: An overpass involves constructing a bridge over the flood-prone area. While this option allows for the passage of floodwater, it may require a significant investment in terms of construction materials and engineering design.
- Underpass: An underpass involves constructing a tunnel beneath the flood-prone area, similar to a subway. It suffers from the same disadvantages as a subway, including higher costs and potential issues with water infiltration.

Conclusion:
In conclusion, an embankment is the most suitable option for building a highway in areas prone to flooding. It provides the necessary elevation, stability, and accessibility while being cost-effective. Other options such as subways, overpasses, or underpasses may have their advantages, but an embankment offers the best combination of functionality and practicality in such situations.