**Core Concept**
Serum ferritin levels are a reliable marker of iron stores in the body, reflecting the amount of iron available for erythropoiesis and other cellular functions. This test is crucial in diagnosing iron deficiency, as it directly correlates with the body's iron reserves.

**Why the Correct Answer is Right**
Serum ferritin is a protein that stores iron in a non-toxic form, and its levels are directly proportional to the body's iron stores. A low serum ferritin level (< 15 ng/mL in non-pregnant women and < 100 ng/mL in men) is indicative of iron deficiency, making it the most sensitive and specific test for diagnosing this condition. This is because serum ferritin is not affected by acute inflammation or other conditions that may alter serum iron levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum iron levels are not a reliable indicator of iron deficiency, as they can be influenced by various factors such as inflammation, infection, and recent iron intake. A low serum iron level may not necessarily reflect the body's iron stores.
**Option C:** Serum transferrin receptor population is a marker of iron deficiency, but it is not as specific as serum ferritin levels. Elevated transferrin receptor levels can also be seen in conditions such as anemia of chronic disease.
**Option D:** Transferrin saturation is the ratio of serum iron to total iron-binding capacity (TIBC) and is an indicator of iron deficiency. However, it is not as sensitive or specific as serum ferritin levels, as it can be affected by various factors such as inflammation and iron overload.

**Clinical Pearl / High-Yield Fact**
Remember that serum ferritin levels are a more reliable indicator of iron stores than serum iron levels, and a value below 15 ng/mL in non-pregnant women or 100 ng/mL in men is indicative of iron deficiency anemia.

**✓ Correct Answer: B. Serum ferritin levels**