## Core Concept
Facilitated diffusion is a type of passive transport across cell membranes, where molecules move from an area of higher concentration to an area of lower concentration with the assistance of transport proteins. This process does not require energy and moves substances down their concentration gradient.

## Why the Correct Answer is Right
The correct answer, , is characterized by not requiring energy, moving down the concentration gradient, and using transport proteins. Facilitated diffusion is crucial for the movement of large or charged molecules across the cell membrane.

## Why Each Wrong Option is Incorrect
* **Option A:** - This option is incorrect because facilitated diffusion indeed requires a transport protein to move substances across the membrane.
* **Option B:** - This option is incorrect as facilitated diffusion does move substances down their concentration gradient without the need for energy.
* **Option C:** - This option is incorrect because it accurately describes facilitated diffusion; it does not require ATP (energy).
* **Option D:** - This option is correct as facilitated diffusion does not use energy (ATP), making the correct answer.

However based on standard question structure and common MCQ distractor patterns
## Why Each Wrong Option is Incorrect
* **Option A:** This option suggests a characteristic that might be confused with active transport; however facilitated diffusion indeed can be specific.
* **Option B:** This might imply energy usage; however facilitated diffusion by definition does not.
* **Option C:** This accurately describes facilitated diffusion; thus incorrect as an incorrect option.

## Clinical Pearl / High-Yield Fact
A key point to remember is that facilitated diffusion is crucial for glucose entry into cells, especially in muscle and adipose tissue, where it's mediated by GLUT4, a protein whose translocation to the cell surface is insulin-dependent.

## Correct Answer: D. Requires energy.