Which of the following is considered an exotic species that has replaced native vegetation?

Hydrilla is considered an exotic species that has replaced native vegetation due to its ability to spread rapidly and outcompete local plant species in aquatic environments. Originally from Africa and Asia, Hydrilla was introduced to the United States as an ornamental plant and has since become invasive in many areas. Its growth is often uncontrolled, leading to significant ecological disruption, including displacement of native aquatic plants, alteration of habitats, and negative impacts on local wildlife. This invasive nature makes it a prime example of an exotic species altering the ecosystem dynamics by undermining native species that are less adaptable to rapid changes in their environment.

In contrast, the other options provided, such as sunflowers, oak trees, and maple trees, are typically native to many regions in North America or can be found in local ecosystems without the same detrimental effects as Hydrilla. Therefore, the inclusion of Hydrilla in the answer highlights its role as an invasive exotic species, which is vital for understanding the impacts of non-native species on native ecosystems.