Turbidity in wastewater is primarily caused by:

Turbidity in wastewater refers to the cloudiness or haziness caused by the presence of suspended particles that scatter light. The primary cause of turbidity is finely divided suspended material. This can include a variety of substances such as silt, clay, organic matter, and other small particles that do not settle easily in water. These suspended materials can significantly affect the water quality and can harbor pathogens, making their presence an essential factor to monitor in wastewater treatment processes.

The other options, while related to water quality, do not directly attribute to turbidity. Color can be a consequence of dissolved organic substances but does not define turbidity itself. Dissolved calcium contributes to overall hardness and impacts calcium levels in water, but it does not cause cloudiness. Hardness primarily relates to the concentration of calcium and magnesium ions and is not a direct contributor to turbidity. Therefore, the correct response emphasizes the importance of finely divided suspended material as the critical factor in determining turbidity in wastewater.