Which feature distinguishes giant water fern from regular water fern?

The correct choice focuses on the unique physical characteristic of the giant water fern, specifically the presence of hairs on the upper leaf surface. This feature sets the giant water fern apart from regular water ferns, as those do not typically exhibit the same hairy texture on their leaves.

The presence of these hairs is significant because it contributes to the plant's ability to regulate water loss and enhance its resilience in aquatic environments. In contrast, the other features listed, such as color, root structure, and height, do not provide the same level of distinction necessary to differentiate the giant water fern from similar species effectively. While other characteristics can vary among aquatic plants, the specific hairiness on the leaf surface provides a clear and identifiable trait for recognizing giant water fern.