**Core Concept**
The extracellular fluid (ECF) compartment in the human body contains a high concentration of electrolytes, primarily sodium, which plays a crucial role in maintaining proper fluid balance and osmotic pressure.

**Why the Correct Answer is Right**
The concentration of sodium in ECF is approximately 135-145 mmol/L, which is maintained by the kidneys through the renin-angiotensin-aldosterone system (RAAS). The RAAS pathway stimulates the release of aldosterone, a steroid hormone produced by the adrenal cortex, which promotes sodium reabsorption in the distal convoluted tubule and collecting ducts of the kidneys. This process helps regulate the amount of sodium in the ECF, ensuring proper fluid balance and blood pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not specify a valid concentration of sodium in ECF.
**Option B:** This option is incorrect because the concentration of sodium in the intracellular fluid (ICF) is much lower, typically around 10-20 mmol/L.
**Option C:** This option is incorrect as it refers to the concentration of sodium in the blood plasma, which is approximately 135-145 mmol/L, but the question asks specifically about the ECF.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the concentration of sodium in ECF can affect the osmotic pressure of the blood, which in turn affects the distribution of water between the ECF and ICF. This is crucial for maintaining proper fluid balance and preventing conditions such as hyponatremia or hypernatremia.

**Correct Answer: D. 135-145 mmol/L**