We have to know which two substances are related correctly.
The correct answer is: (C) H₃O⁺ is the conjugate acid of H₂O.
In an acid-base reaction, an acid reacts with a base and gives a conjugate base and conjugate acid. The reaction is shown below:
Acid₁ + Base₂ ⇄ Conjugate Base₁ + Conjugate Acid₂
In the reaction H₂CO₃ + H₂O ⇌ H₃O⁺ + HCO₃⁻, H₂CO₃ is an acid because it releases H⁺ ion and converts to HCO₃⁻. Here HCO₃⁻ is the conjugate base of H₂CO₃ ( according Arrhenius theory).
H₂O accepts H⁺ ion and is converted to H₃O⁺ , thus H₂O behaves as Bronsted base. So, H₃O⁺ is the conjugate acid of Bronsted base H₂O.
Hence, the correct answer is: (C) H₃O⁺ is the conjugate acid of H₂O.