Option-wise explanation

1. Eichornia (water hyacinth) – correct

• Eichhornia crassipes is a free‑floating aquatic macrophyte with a large root system that absorbs and accumulates heavy metals (Pb, Cd, Cu, Zn, As, etc.) from wastewater and surface waters.​

• It is one of the most commonly cited plants for phytoremediation of heavy‑metal‑contaminated water, so exam questions usually take this as the standard answer.

2. Nymphia (Nymphaea / water lily)

• Some Nymphaea species can take up metals like Cd and Cr from water and sludge and have shown phytoremediation potential.​

• However, they are less commonly highlighted in introductory questions than water hyacinth.

3. Pistia (water lettuce)

• Pistia stratiotes can also remove metals such as Pb, Cr, Ni from water via rhizofiltration, and many studies support its phytoremediation potential.​

• Again, in exam practice, Eichhornia is the more “classic” answer for heavy‑metal removal from water.

4. Salvia

• Salvia (clary sage, etc.) is mostly a terrestrial aromatic/medicinal genus; some species tolerate or accumulate metals in soils, but they are not standard aquatic plants for removing metals from water bodies.​

Thus, among the options, the plant most characteristically used and asked about for removal of heavy metals from water is Eichhornia (water hyacinth) — option (1).