**Core Concept**
Plasma levels of human chorionic gonadotropin (hCG) are a critical marker of pregnancy, reflecting trophoblastic function and placental health. Understanding hCG dynamics is essential for monitoring fetal well-being and diagnosing potential complications.

**Why the Correct Answer is Right**
hCG levels in pregnancy exhibit a characteristic doubling time, which is a crucial parameter for clinical assessment. The doubling time of hCG is approximately 48 hours, reflecting the rapid growth and development of the trophoblastic cells that produce this hormone. This exponential increase in hCG production is a hallmark of healthy placental function and is used as a prognostic indicator in early pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Daily doubling of hCG would imply an excessively rapid increase, potentially indicating a molar pregnancy or other pathologic conditions. This rate of increase is not physiologically normal.
**Option C:** A doubling time of 4 days would suggest a slower rate of trophoblastic growth, which may be seen in cases of placental dysfunction or abnormal implantation.
**Option D:** A doubling time of 6 days would be significantly longer than expected, potentially indicating a failure of trophoblastic function or an abnormal pregnancy.

**Clinical Pearl / High-Yield Fact**
A doubling time of less than 48 hours is often associated with an increased risk of multiple gestations, while a doubling time of more than 72 hours may indicate potential complications such as ectopic pregnancy or miscarriage.

**✓ Correct Answer: B. 2 days**