Correct option (b) Volvox → Hydrilla → Pistia → Scirpus → Lantana → Oak
The various stages in a hydrosere are well studied in ponds, pools or lakes. The 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. Nelumbo, Nymphaea, etc. Some free floating species are pistia, Azolla, Lemna, etc. (iv) Red-swamp stage, e.g. species of Scirpus, Typha. etc. (v) Sedge-meadow stage, e.g. species of Cyperaceae and Gramineae. (vi) Woodland stage, e.g. Lantana, Salix, Populus, etc (vii) Forest stage, e.g. Tropical rain forests, mixed forests of Almus, Acer, Quercus (oak), tropical deciduous forests.