(C) is the most likely to donate a proton because loss of a proton allows the pyrrole to become aromatic by freeing up the lone pair of the nitrogen and allowing it to delocalize.
The least aromatic should be (B) since the nitrogen is attached to two alkyl groups which act as electron donating groups. Since these alkyl groups donate electron density, they help stabilize the positive charge on the nitrogen. On the other hand, (A) is a primary amine, so there is only one electron donating group attached. Therefore, the positive charge on the nitrogen in (A) is stabilized less than the positive charge on the nitrogen in (B).