**Core Concept**
The best position for detecting pleural effusions on chest X-ray is crucial in radiology, as it affects the sensitivity and specificity of the diagnosis. The erect position, also known as the upright position, is preferred for detecting left-sided pleural effusions due to the effects of gravity on fluid distribution.

**Why the Correct Answer is Right**
In the erect position, gravity causes the pleural effusion to accumulate in the dependent portion of the lung, which is the lower lobe in the case of a left-sided effusion. This position allows for the detection of even small amounts of fluid, making it ideal for identifying pleural effusions. In contrast, the supine position can make it difficult to detect pleural effusions, as the fluid accumulates in the anterior portion of the lung, which may not be visible on the X-ray.

**Why Each Wrong Option is Incorrect**
* **Option A:** The left lateral decubitus position is not the best choice for detecting left-sided pleural effusions. While it can help to visualize small amounts of fluid, it may not be as effective as the erect position.
* **Option B:** The supine position is not ideal for detecting left-sided pleural effusions, as mentioned earlier, as the fluid accumulates in the anterior portion of the lung, making it difficult to visualize.
* **Option C:** The prone position is not typically used for detecting pleural effusions on chest X-ray. It may actually make it more difficult to visualize the effusion due to the positioning of the lungs and diaphragm.

**Clinical Pearl / High-Yield Fact**
Remember the "E" in "Erect" stands for "Effusion" - the erect position is the best choice for detecting pleural effusions on chest X-ray.

**Correct Answer: C. Supine position.