Hardness is attributed to multivalent cations, principally calcium and magnesium, not the carbonate anion itself. Carbonate can accompany hardness-forming cations but does not supply cationic hardness. Thus official **D**. The printed $Si^{2+}$ option is chemically unusual for ordinary water; it is retained as printed rather than silently replaced with another divalent metal.
More Water and Wastewater Quality questions
GATE 2024 ES Q46– Which are Streeter–Phelps oxygen-sag assumptions?
GATE 2023 ES Q58– A BOD bottle contains 25 mL wastewater diluted to 300 mL. Wastewater ultimate BOD75 mg/L,…
GATE 2022 ES Q45– Rivers X, Y have reaeration constants .92,1.12 day⁻¹ and deoxygenation constants .23,.35…
GATE 2026 ES Q30– Total Kjeldahl Nitrogen includes which forms of nitrogen? Select all that apply.
GATE 2026 ES Q41– A water sample has Na+,Ca2+,Mg2+,Cl-,SO 42-,NO 3- concentrations 30,10,15,30,60,5 mg/L…