**Core Concept**
Lignocaine is a local anesthetic drug that is commonly used in various medical procedures. The concentration of lignocaine in a solution is usually expressed in terms of weight per unit volume, typically in grams per milliliter (g/mL). The amount of lignocaine present in a solution can be calculated using the formula: Amount of lignocaine = Concentration of lignocaine x Volume of solution.

**Why the Correct Answer is Right**
To calculate the amount of lignocaine present in 2.0 mL of 2% lignocaine solution, we need to first convert the concentration of lignocaine from percentage to grams per milliliter. Since 2% lignocaine solution is equivalent to 2 g of lignocaine per 100 mL of solution, we can calculate the concentration as follows: Concentration of lignocaine = 2 g / 100 mL = 0.02 g/mL. Now, we can use the formula to calculate the amount of lignocaine present in 2.0 mL of solution: Amount of lignocaine = Concentration of lignocaine x Volume of solution = 0.02 g/mL x 2.0 mL = 0.04 g.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not take into account the concentration of lignocaine in the solution. Without knowing the concentration, we cannot calculate the amount of lignocaine present in the solution.
**Option B:** This option is incorrect because it does not use the correct units for concentration and volume. Lignocaine concentration is typically expressed in grams per milliliter (g/mL), not milligrams per milliliter (mg/mL).
**Option C:** This option is incorrect because it does not use the correct formula to calculate the amount of lignocaine present in the solution. The correct formula is Amount of lignocaine = Concentration of lignocaine x Volume of solution.
**Option D:** This option is incorrect because it does not give a numerical value for the amount of lignocaine present in the solution.

**Clinical Pearl / High-Yield Fact**
When calculating the amount of a drug present in a solution, it's essential to use the correct units for concentration and volume, and to apply the correct formula to ensure accurate calculations.

**Correct Answer: A. 0.04 g.**