Step 1: Vasodilator Identification
Vasodilators are agents that expand blood vessels, thereby lowering blood pressure.
Histamine is a significant mediator in immune responses and vasodilation.
Step 2: Analysis of Provided Choices
Histamine induces vasodilation and controls gastric acid production.
Serotonin influences mood and coagulation but does not act as a vasodilator.
Codeine functions as an opioid analgesic.
Cimetidine operates as a histamine receptor antagonist.
Conclusion: Given that histamine is the principal vasodilator, option (A) is identified as correct.