Which of the following is a true statement about algae?

Algae are a diverse group of photosynthetic organisms that can be found in various environments, not just freshwater. They can exist as planktonic organisms that float in water, filamentous forms that can be stringy or thread-like, and erect forms that might grow upright from substrates. Understanding this diversity in morphology is essential when managing algal populations in different aquatic environments.

The other statements, while touching on aspects of algae, do not accurately reflect the complete nature of these organisms. They are not limited to freshwater environments, as some thrive in marine settings as well. Algae are classified based on several criteria, including their photosynthetic pigments, cell wall composition, and habitat, rather than strictly by color. While herbicides can be used for managing certain types of problematic algae, there are multiple management strategies available, which include physical removal and nutrient control. Thus, statement C correctly captures a significant aspect of algae, recognizing their varied forms.