Step 1: What "vasodilator" means at the receptor level.
A vasodilator loosens the smooth muscle ringing a blood vessel by triggering nitric oxide release or directly relaxing that muscle, so the vessel widens and local blood flow goes up.
Step 2: Check histamine against that test.
Histamine binds H1 receptors on the endothelium lining vessels. That binding switches on nitric oxide production, which relaxes the surrounding smooth muscle, so histamine passes the vasodilator test.
Step 3: Check bradykinin against the same test.
Bradykinin works through B2 receptors on endothelial cells, and this pathway also releases nitric oxide and prostacyclin. Both are strong relaxants of vascular smooth muscle, so bradykinin passes too.
Step 4: Rule out the other two compounds together.
Heparin's job is blocking the clotting cascade, it has no receptor action on vessel wall muscle, so it never enters the vasodilator category. Serotonin, in most vascular beds, actually tightens smooth muscle through 5-HT2 receptors, giving vasoconstriction rather than dilation, the opposite of what the question asks for.
Step 5: Match to the options.
Only the pair histamine and bradykinin both satisfy the receptor test for dilation. Any option pairing heparin or serotonin fails on at least one compound.
Final Answer:
Histamine and bradykinin constitute the vasodilators, option (4).