The conjugate base of an acid is simply the original acid LESS a proton, H+. And likewise the conjugate acid of a base is the original base PLUS a proton. As with any chemical process, both mass and charge are conserved.
So for a. we have the acid NH+4, whose conjugate base is NH3. And we have the base, −C≡N, whose conjugate acid is HC≡N.
And for b. we have the acid HCl(aq), whose conjugate base is Cl−. And we have the base, CO2−3, whose conjugate acid is HCO−3, bicarbonate ion.
And for c. we have the acid HCl, whose conjugate base we have already identified.
Note that all I have done here is to add (conjugate acid) or subtract (conjugate base) a proton, and conserved charge.
For water, H2O, the conjugate acid is H3O+, hydronium ion. Its conjugate base is HO−, hydroxide ion. And the conjugate base of HO−≡O2−.
And if we go to ammonia as a SOLVENT, we can invoke equivalent conjugate acid/base pairs for NH+4, and NH−2 (this amide base is TOO BASIC to exist in water).
Confused yet?
[And see here and links.](https://socratic.org/questions/what-is-the-conjugate-acid-base-relationship-of-h2po4-and-hpo4)