**Core Concept**
Lignocaine is a local anesthetic agent, and its concentration in a solution is typically expressed as a percentage of weight per volume (e.g., 2% lignocaine means 2 grams of lignocaine per 100 milliliters of solution). To determine the amount of lignocaine in a specific volume, we need to calculate the concentration in milligrams per milliliter.

**Why the Correct Answer is Right**
To find the concentration of lignocaine in 1 ml of a 2% solution, we first need to determine the total amount of lignocaine in the 4 ml vial. Since the solution is 2% lignocaine, we calculate the total amount as follows: 4 ml x 2 g/100 ml x (1000 mg/1 g) = 80 mg. To find the concentration in 1 ml, we divide this total amount by 4 ml: 80 mg / 4 ml = 20 mg/ml.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the concentration of lignocaine in a 2% solution is 2 g/100 ml, not 2 mg/ml.
**Option B:** This option is incorrect because it incorrectly doubles the concentration of lignocaine in a 2% solution.
**Option D:** This option is incorrect because it is an order of magnitude larger than the actual concentration of lignocaine in a 2% solution.

**Clinical Pearl / High-Yield Fact**
When working with local anesthetic solutions, it's essential to understand the concentration and volume to accurately determine the dose. Always double-check the concentration and volume before administering local anesthetics to avoid overdose or underdose.

**✓ Correct Answer: C. 20 mg**