Which feature can create a round swept profile with only a guide string?

The correct choice for creating a round swept profile with only a guide string is the Tube feature. This feature specifically allows the creation of tubular shapes by defining a cross-sectional profile that is swept along a given path or guide string. The Tube operation focuses on generating cylindrical shapes based on the provided guide, making it particularly effective for creating tubes, pipes, or other hollow structures.

In contrast, other options like Sweep along a guide and Swept generally require defining both a profile and additional parameters to specify the sweep. Variational sweep, while it can create complex shapes, also typically involves multiple guiding curves or additional control constraints that exceed the simplicity of a single guide string. Thus, the Tube feature stands out for its direct approach in using just a guide to form rounded profiles efficiently.