Which of the following plants is mostly submerged and forms daughter plants from seeds germinating on the mother plant?

The correct choice highlights the characteristics of the Slender spikerush (typically found in aquatic environments) where it predominantly grows submerged in water. This plant is notable for its ability to form daughter plants—a reproductive strategy that allows for the propagation of itself. The germination of seeds on the mother plant contributes to the development of new plants in close proximity to the parent, enhancing its capability to thrive in aquatic systems.

In contrast, the other options listed do not share this specific feature of forming daughter plants by germinating seeds on the mother plant. For instance, Soft rush is typically found in moist soils and has a different growth form without submerged characteristics. Torpedograss is primarily a terrestrial grass that invades wetlands but does not utilize submerged propagation in the same manner. Water fern, while also adapted to aquatic habitats, reproduces primarily through vegetative means and spores rather than through seeds germinating directly on the parent plant.

Understanding the unique reproductive mechanisms and growth forms of these plants is crucial in the context of aquatic weed control, as it informs management strategies for those species that can proliferate rapidly in aquatic environments.