According to Bronsted-Lowery theory of acid and base, those compounds which donates a proton are called acid and compounds which accept proton are called base. The species after donating a proton is called conjugate base of that acid and conjugate base of strong acid is weak and of weak acid is strong. Similarly species formed after receiving a proton is called conjugate acid. Conjugate acid of weak base is strong and that of strong base is weak. In the given question acetylene is a weak acid. So, its conjugate base i.e., H−C≡C will be strong.