**Core Concept**
The size of an intravenous (i.v.) cannula is often indicated by its color, which signifies the gauge (thickness) of the cannula. A green-colored i.v. cannula typically indicates a smaller gauge, which is associated with a larger size in terms of the French scale used to measure cannula diameters.

**Why the Correct Answer is Right**
The correct answer for a green-colored i.v. cannula is often 16G (gauge). This is because the French scale is inversely related to gauge, meaning that a smaller gauge corresponds to a larger French size. A 16G cannula is relatively large in size, making it suitable for larger veins or for patients with poor peripheral circulation.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the size and gauge of the cannula are not directly related to its color. Different colors may indicate different sizes or types of cannulas.
* **Option B:** This option is incorrect because the size of the cannula is not necessarily related to its color. Without more information about the specific color, it's difficult to determine the correct size.
* **Option C:** This option is incorrect because the size and gauge of the cannula are not directly related to its color. Different colors may indicate different sizes or types of cannulas.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the size and gauge of an i.v. cannula can affect the flow rate of fluids and medications, as well as the risk of complications such as phlebitis or extravasation. Choosing the correct size of cannula is crucial to ensure patient safety and effective treatment.

**Correct Answer: D. 16G**