Here's why:

* Conjugate base: A conjugate base is formed when an acid loses a proton (H⁺).

* HC₂O₄⁻ as an acid: HC₂O₄⁻ can act as an acid because it can donate a proton.

* Losing a proton: When HC₂O₄⁻ loses a proton, it becomes C₂O₄²⁻.

Therefore, C₂O₄²⁻ is the conjugate base of HC₂O₄⁻.