The second stage of hydrosere is occupied by plants like Typha, Salix, Vallisneria, Azolla

The Hydrosere succession is the succession in which, a pond and its community are converted into land community. Various stages of hydrosere are:
(i) Phytoplankton stage: e.g., some blue-green algae, green algae (Volvox), diatoms and bacteria etc
(ii) Rooted submerged stage, e.g., Hydrilla, Vallisneria, etc
(iii) Floating stage, e.g., Nymphaea, Some free floating species like Azolla, Pistia etc
(iv) Reed swamp stage, e.g., species of Typha
(v) Sedge-meadow stage, e.g., species of Cyperaceae and Gramineae
(vi) Woodland stage, e.g., Lantana, Salix, Populus
(vii) Forest stage, e.g., Tropical rainforests, tropical deciduous forests.
Hence, the second stage of hydrosere is occupied by Vallisneria. So, the correct answer is 'Vallisneria'.