**Core Concept**
The lifespan of fetal red blood cells (RBCs) is a critical aspect of fetal physiology, particularly in understanding the differences between fetal and adult erythropoiesis. Fetal RBCs play a vital role in oxygen delivery to the developing fetus and have distinct characteristics compared to adult RBCs.

**Why the Correct Answer is Right**
Fetal RBCs have a shorter lifespan compared to adult RBCs due to the presence of a higher concentration of fetal hemoglobin (HbF), which is more prone to oxidative stress and cellular destruction. This shorter lifespan also allows for a more rapid turnover of RBCs, enabling the fetus to adapt to changes in oxygen supply and demand. The average lifespan of fetal RBCs is approximately 80-90 days.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a specific lifespan for fetal RBCs.
**Option B:** This option is incorrect because it suggests a lifespan that is too long for fetal RBCs.
**Option C:** This option is incorrect because it is not the correct lifespan for fetal RBCs.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the lifespan of fetal RBCs is shorter than that of adult RBCs, which is approximately 120 days. This difference highlights the unique physiological demands of fetal development and the need for a more rapid turnover of RBCs.

**Correct Answer:** C. 80-90 days.