Heparin and histamine from mast cells function to directly cause
a. vasodilation.
b. vasoconstriction.
c. neutrophils to kill invading microorganisms.
d. conversion of fibrinogen to fibrin.

I believe is to directly cause vasodilation. Mast cells are cells found in connective tissue that contain numerous basophillic granules and releases substance such as heparin and histamine in response to injury or inflammation of bodily tissue. Heparin is normally an anticoagulant which prevents blood from clotting too quickly while histamine is a vasodilator which promotes blood flow to tissues. Vasodilation is the widening of blood vessels which results from relaxation of smooth muscles within the vessels walls.