In which of the following is the nitrogen lone pair a part of the aromatic pi sextet (so that it is not the site of protonation)?

In pyrrole and at the N-H nitrogen of imidazole, the lone pair is donated into the ring to give six pi electrons. In pyridine and at N-3 of imidazole the nitrogen has a double bond in the ring, contributing one p electron, and the lone pair is in an in-plane sp${}^2$ orbital available for protonation.