The correct answer is option 'B' - Drainage basin.

A drainage basin, also known as a river basin or catchment area, refers to the area of land that is drained by a single river system and its tributaries. It is the region where all the precipitation, such as rain, snow, and ice melt, is collected and channeled into a common outlet, which is usually a river or a lake. The concept of a drainage basin is important in hydrology and geography as it helps in understanding the movement and distribution of water on the Earth's surface.

Key Points:
1. Area Drained by a Single River System: A drainage basin encompasses all the land and water bodies that contribute water to a particular river system. It includes the area where rainwater falls and eventually flows into the river, as well as the surrounding land that contributes runoff and groundwater to the river.

2. Collection of Precipitation: The drainage basin acts as a natural collector or reservoir for precipitation. When rain falls or snow melts within the boundaries of the basin, the water is absorbed into the ground, flows over the land surface, or seeps into groundwater aquifers. All of these water sources eventually make their way into the river system.

3. River as the Outlet: The river within the drainage basin serves as the main outlet for the collected water. It gathers water from its tributaries, which are smaller rivers or streams that flow into the main river. As the river flows through the basin, it carries the water collected from various sources and transports it to the sea, a lake, or another river system.

4. Dividing Lines: The boundaries of a drainage basin are determined by the topography of the land. These boundaries are often defined by natural features such as mountain ranges or ridges, which act as divides between different river systems. The line separating one drainage basin from another is known as a drainage divide or watershed.

Conclusion:
In conclusion, the area drained by a single river system is called a drainage basin. It encompasses all the land and water bodies that contribute water to the river, and it plays a crucial role in the movement and distribution of water on Earth's surface.