Which of the following is a typical feature of filamentous algae?

Filamentous algae are indeed characterized by features that align with the correct choice. A typical feature of filamentous algae is that they often lack true roots. Instead of having roots like higher plants that anchor them to the substrate, filamentous algae typically attach themselves to surfaces using a holdfast mechanism or float freely in the water column. This absence of roots is significant because it allows them to thrive in various aquatic environments, whether they are attached to rocks or suspended in the water.

The other options mentioned do not accurately reflect the typical characteristics of filamentous algae. For instance, while many filamentous algae may be submerged, there are species that grow partially above the water level, which makes the statement about being completely submerged misleading. Additionally, although they are capable of standing upright due to the buoyancy provided by water, they are not considered self-supporting plants in the same way that higher plants are since they lack a complex structure with supportive tissues. Filamentous algae may be found in various water conditions, but they do not have a universal preference for fast-moving currents; they can inhabit both stagnant and flowing waters, contradicting the assertion of a specific preference for flow.