**Core Concept**
The lifespan of fetal red blood cells (RBCs) is a critical aspect of fetal physiology, particularly during pregnancy. Fetal RBCs are responsible for transporting oxygen from the mother's bloodstream to the fetus, and their lifespan is influenced by various factors, including the mother's health and the presence of any underlying medical conditions.

**Why the Correct Answer is Right**
Fetal RBCs have a longer lifespan compared to adult RBCs due to their unique characteristics. They have a larger size and a more flexible membrane, which allows them to withstand the mechanical stresses of fetal circulation. Additionally, fetal RBCs have a higher concentration of 2,3-bisphosphoglycerate (2,3-BPG), an enzyme that helps regulate oxygen delivery to tissues. This adaptation enables fetal RBCs to maintain efficient oxygenation of the fetus despite the limited oxygen availability in the maternal bloodstream.

**Why Each Wrong Option is Incorrect**
* **Option A:** is incorrect because it does not accurately reflect the lifespan of fetal RBCs. While adult RBCs typically have a lifespan of around 120 days, fetal RBCs have a shorter lifespan.
* **Option B:** is incorrect because it is not a well-established estimate of the lifespan of fetal RBCs. The actual lifespan of fetal RBCs is shorter than that of adult RBCs.
* **Option C:** is incorrect because it is not supported by scientific evidence. The lifespan of fetal RBCs is not influenced by the mother's age.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the lifespan of fetal RBCs can be affected by various maternal and fetal conditions, such as anemia, hypertension, and intrauterine growth restriction. Monitoring the lifespan of fetal RBCs can provide valuable insights into the health and well-being of both the mother and the fetus.

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