Based on the Bronsted-Lowry theory:
An acid is defined as a species which in solution tends to lose a proton (H+) while base is a species that tends to accept protons.
The given situation, HSO4⁻ acts as an acid and loses its H+ to CN⁻.
HSO4⁻ + CN⁻↔ SO4²⁻ + HCN
Here, HSO4⁻ is the acid while SO4²⁻ is its conjugate base
CN⁻ is a base and HCN is its conjugate acid
