Baseflow refers to the delayed flow that reaches the river mainly as groundwater. It is an important component of the hydrological cycle and plays a crucial role in maintaining streamflow during dry periods. Here is an explanation of why the correct answer is option 'C' - Baseflow.

Explanation:
1. Baseflow definition:
- Baseflow is the portion of streamflow that comes from the slow seepage of water from the subsurface, primarily from groundwater.
- It is called baseflow because it forms the base or foundation of streamflow, providing a consistent flow even during dry periods.

2. Sources of baseflow:
- Baseflow is mainly derived from groundwater that has infiltrated the soil and gradually makes its way to the river.
- It represents the delayed flow component of the hydrological cycle, where water moves slowly through the subsurface before reaching the river.
- Other sources of baseflow can include springs, seepage from lakes or reservoirs, and groundwater discharge from wetlands.

3. Characteristics of baseflow:
- Baseflow has a relatively constant flow rate compared to surface runoff, which can vary greatly during storms or wet periods.
- It contributes to the overall streamflow regime, ensuring a sustained flow even when there is no rainfall or runoff.
- Baseflow can have a cooling effect on river water temperatures due to its lower temperature compared to surface runoff.

4. Importance of baseflow:
- Baseflow is essential for maintaining ecological health in rivers and streams, providing a constant water supply for aquatic organisms during dry periods.
- It helps sustain water availability for human use, such as drinking water supplies, irrigation, and industrial processes.
- Baseflow also contributes to the dilution of pollutants in rivers, as groundwater often undergoes natural filtration processes.

In conclusion, baseflow refers to the delayed flow that reaches the river mainly as groundwater. It is an important component of streamflow, providing a consistent and sustained water supply during dry periods.