**Core Concept**
Surfactant is a complex mixture of lipids and proteins that reduces surface tension in the lungs, facilitating breathing and preventing lung collapse. Its presence in amniotic fluid is a crucial indicator of fetal lung maturity.

**Why the Correct Answer is Right**
Surfactant production in the fetal lungs begins around 24 weeks of gestation, but it takes some time for the surfactant to be secreted into the amniotic fluid. By 32 weeks, the majority of fetuses have developed sufficient lung maturity, as indicated by the presence of surfactant in the amniotic fluid. This is because surfactant is produced by type II pneumocytes in the lung, and the process of surfactant maturation is closely linked to the development of the fetal lung.

**Why Each Wrong Option is Incorrect**
**Option A:** 20 weeks is too early for surfactant to be present in amniotic fluid, as lung maturity and surfactant production are still in the developmental stages.

**Option D:** 26 weeks is still relatively early for widespread surfactant presence, although some fetuses may start to show surfactant in their amniotic fluid.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the presence of surfactant in amniotic fluid is a key indicator of fetal lung maturity, but it's not the only factor. Other markers, such as the lecithin-sphingomyelin ratio (L/S ratio), can also be used to assess lung maturity.

**✓ Correct Answer: B. 32 weeks**