**Core Concept**
Infants undergo a gradual transition from fetal hemoglobin (HbF) to adult hemoglobin (HbA) as they mature, a process crucial for optimal oxygen delivery and overall health.

**Why the Correct Answer is Right**
This transition is mediated by the gradual decline of erythropoietin (EPO), a hormone that promotes the production of red blood cells, and the increase in the production of adult hemoglobin chains. The switch from HbF to HbA is largely complete by 6-9 months of age, allowing infants to adapt to the increased oxygen demands of growth and development. The process involves the degradation of HbF by the enzyme dipeptidyl peptidase-2 (DPPII) and the increased production of HbA chains.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not specify a time frame, making it a vague and incorrect answer.
**Option B:** This option is incorrect because the transition from fetal to adult hemoglobin is not complete at birth.
**Option C:** This option is incorrect because the switch from HbF to HbA is not a sudden event, but rather a gradual process that occurs over several months.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the transition from fetal to adult hemoglobin is a critical process in infant development, and any disruptions to this process can lead to anemia or other blood disorders.

**Correct Answer: B. 2 years.**