**Compact Settlements in River Valleys and Plains**

**Introduction:**
Compact settlements refer to the clustering of houses and buildings in a small area, resulting in a high population density. These settlements are generally found in specific geographical regions that offer favorable conditions for human habitation. Among the given options, river valleys and plains are the most suitable areas for compact settlements. This answer will provide a detailed explanation of why compact settlements are found in river valleys and plains.

**1. Availability of Water Resources:**
- River valleys and plains often have abundant water resources in the form of rivers, lakes, and streams.
- Water is essential for various purposes such as drinking, cooking, agriculture, and livestock rearing.
- The availability of water resources encourages human settlement in these areas, leading to compact settlements.

**2. Fertile Soil and Agriculture:**
- River valleys and plains are known for their fertile soil, which is ideal for agricultural activities.
- The flat terrain and alluvial deposits from rivers make the soil rich in nutrients, supporting the growth of crops.
- Compact settlements in these areas are clustered around agricultural fields, allowing easy access to farmland and promoting efficient agricultural practices.

**3. Transportation and Trade:**
- River valleys and plains offer favorable conditions for transportation and trade.
- Rivers provide natural transportation routes, enabling the movement of goods and people.
- Compact settlements in these areas are strategically located along rivers, facilitating trade and commerce.
- River valleys and plains often serve as commercial centers, attracting traders and merchants, thereby promoting compact settlements.

**4. Floodplains and Irrigation:**
- River valleys and plains experience periodic flooding, which can be advantageous for agriculture.
- Floodplains deposit fertile sediments, enriching the soil and supporting agricultural productivity.
- Compact settlements in these areas are designed to utilize floodplains for agriculture through irrigation systems and canals.

**Conclusion:**
Compact settlements are primarily found in river valleys and plains due to the availability of water resources, fertile soil for agriculture, transportation opportunities, and the utilization of floodplains for irrigation. These factors make river valleys and plains highly suitable for human habitation and promote the clustering of houses and buildings in a small area, resulting in compact settlements.