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 NH_3. 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, CO_3^(2-), 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, H_2O, the conjugate acid is H_3O^+, "hydronium ion". Its conjugate base is HO^-, "hydroxide ion". And the conjugate base of HO^(-)-=O^(2-).
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)
