**Core Concept**
The question tests the knowledge of additives used to enhance the properties of local anesthetics. The underlying principle is that certain agents can modify the pharmacokinetics or pharmacodynamics of local anesthetics to achieve faster onset of action.

**Why the Correct Answer is Right**
The correct answer is epinephrine (adrenaline). Epinephrine works by causing vasoconstriction, which reduces blood flow to the area and slows the removal of the local anesthetic from the site of action. This results in a higher concentration of the anesthetic at the nerve terminals, leading to a faster onset of action.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cocaine is a local anesthetic itself, not an additive used to speed the onset of action.
* **Option B:** Dibucaine is a long-acting local anesthetic, not an additive used to enhance the onset of action.
* **Option C:** Lignocaine (lidocaine) is a common local anesthetic, but it is not used as an additive to speed the onset of action.

**Clinical Pearl / High-Yield Fact**
When using epinephrine as an additive to local anesthetics, it is essential to note that it can cause vasoconstriction, which may lead to tissue ischemia if the area is small or the dose of epinephrine is excessive.

**Correct Answer: C. Epinephrine.