Final answer:
In the chemical equilibrium equation given, CN− accepts a proton to become HCN, which makes HCN the conjugate acid according to the Brønsted-Lowry definition.
Step-by-step explanation:
According to the Brønsted-Lowry acid-base theory, a conjugate acid is the species that forms when a base accepts a proton (H+), while a conjugate base is what remains of the acid after it donates a proton. In the provided chemical equilibrium equation CN− + H2CO3 ⇌ HCN + HCO3−, CN− is acting as a Brønsted-Lowry base (BB) as it accepts a proton becoming HCN, which is its conjugate acid (CA). Therefore, the correct answer to which species is the conjugate acid is C: HCN.