Which ornamental stem plant is commonly grown in ponds and aquatic plant systems?

Ceratophyllum demersum, commonly known as coontail or hornwort, is the correct choice as it is a highly adaptable and robust aquatic plant that thrives in pond environments and various aquatic plant systems. This submerged plant does not have true roots, which allows it to float freely and absorb nutrients from the water column. It plays an essential role in providing oxygen to the water, serving as habitat for aquatic organisms, and controlling algae growth.

Other choices like Bacopa and Java Fern are also aquatic plants but are typically found in different settings or have specific care requirements that can limit their effectiveness in larger pond systems. Additionally, while Sword plants are popular in aquariums, they usually need specific substrate conditions for their roots, making them less common in extensive natural pond settings compared to Ceratophyllum demersum. This adaptability and its role in pond ecosystems highlight why Hornwort is the optimal candidate for growth in such environments.