The predominant form of histamine present in human blood is:[pKₐHistidine=6.0]

pKa of N of imidazole is 6 (from histidine pKa) and that of NH₂ is 9.7. pH of blood is 7.4. Since the pH is lower than the pKa of NH₂, then the compound will be protonated at NH₂ and neutral at N of imidazole. Hence, the correct option is B