**Core Concept**
The question is testing the concept of pediatric dosing, specifically the calculation of drug doses in children based on their weight or body surface area. This is an essential principle in pediatrics, as it ensures that children receive appropriate and safe doses of medications.

**Why the Correct Answer is Right**
To determine the dose of a drug in a 3-year-old child, we use the weight-based dosing approach. The typical method is to calculate the dose based on the child's weight in kilograms. For most medications, the dose is usually calculated as a fraction of the adult dose, often around 10-20 mg/kg or 2-4 mg/kg, depending on the specific medication. In this case, let's assume the correct dose is 15 mg/kg.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it doesn't provide a dose calculation method or a specific dose value, making it an unsuitable choice for a pediatric dosing scenario.
**Option B:** This option is incorrect because it suggests a fixed dose for all children, regardless of their weight or age, which is not a standard practice in pediatric dosing.
**Option C:** This option is incorrect because it implies a dose calculation based on the child's age rather than weight, which is not an accurate method for pediatric dosing.

**Clinical Pearl / High-Yield Fact**
When prescribing medications to children, it's essential to use weight-based dosing to avoid overdosing or underdosing. The ideal dose can be calculated using the child's weight in kilograms and the adult dose as a reference.

**Correct Answer: C. 10 mg/kg**