Which of the following describes a feature of algae?

The correct choice highlights that algae are photosynthetic organisms capable of existing as single cells or forming colonies. This characteristic is fundamental to understanding algae because they can exist in various forms, from unicellular species, like Chlamydomonas, to multicellular forms, such as seaweeds. The capacity for photosynthesis allows algae to convert light energy into chemical energy, which supports their growth in diverse aquatic environments.

In contrast, the options relating to being present only in freshwater, being solely multicellular, or requiring sunlight without water misrepresent essential aspects of algae biology. Algae can be found in both freshwater and marine environments, not limited to one. They are not exclusively multicellular; many species are single-celled. Additionally, their growth depends on both sunlight and water, as water is essential for photosynthesis and nutrient uptake. Thus, the description of algae as versatile photosynthetic organisms accurately captures their significance in aquatic ecosystems.