Final answer:
A vasoconstrictor is added to local anesthetics to slow absorption into the bloodstream, thus extending the anesthetic's duration of action and reducing bleeding in the treated area.
Step-by-step explanation:
A vasoconstrictor is added to local anesthetic agents to slow down the intake of the agent and increase the duration of action. When a vasoconstrictor such as epinephrine is added to a local anesthetic, it causes the blood vessels in the area to constrict. This decreases blood flow to the area, which slows the absorption of the anesthetic into the bloodstream and therefore prolongs its effect. The vasoconstriction also helps to reduce bleeding in the area where the local anesthetic is administered.
The correct answer is C) Vasoconstrictor.