For the given compound, which of the indicated nitrogen atoms has the least tendency to be protonated?

Step-by-step solution

In the given compound, the nitrogen atoms indicated by arrows 'b', 'c', and 'd' are pyridine-like nitrogens. They are part of double bonds, and their lone pairs reside in sp^2 hybridized orbitals. These orbitals are orthogonal to the -system of the rings, meaning the lone pairs are localized and readily available to accept a proton. The nitrogen atom indicated by arrow 'a' is a pyrrole-like nitrogen. It is bonded to a hydrogen atom and two carbon atoms via single bonds. Its lone pair resides in an unhybridized p-or