Final answer:
Vasoconstrictors are added to local anesthetics primarily to increase the duration of the anesthetic effect by reducing systemic absorption, thus also minimizing potential toxicity. Option a.
Step-by-step explanation:
The primary reason that vasoconstrictors are added to local anesthetics is to increase the duration of action option a. This effect is achieved because vasoconstrictors narrow the blood vessels, which reduces the rate of systemic absorption of the anesthetic. This slower absorption not only prolongs the anesthetic effect at the target site by keeping the anesthetic localized for longer but also minimizes the potential for toxicity since there is less anesthetic entering the systemic circulation at once. Moreover, by delaying systemic absorption, lower doses of local anesthetics can often be used, further reducing the risk of systemic side effects. Vasoconstrictors do not primarily increase the affected area of the anesthetic, and their role is not to decrease the duration of action.