**Core Concept**
Surfactant is a complex mixture of lipids and proteins that reduces surface tension in the lungs, facilitating easier lung expansion and preventing alveolar collapse during exhalation. 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 24-28 weeks of gestation, but its appearance in amniotic fluid is a more reliable indicator of lung maturity. The presence of surfactant in amniotic fluid suggests that the fetal lungs are capable of producing and secreting surfactant, which is essential for postnatal breathing. This is often used as a marker to predict the likelihood of respiratory distress syndrome (RDS) in preterm infants.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, please refer to the correct answer.
**Option B:** This is incorrect as surfactant appears in amniotic fluid at a later gestational age than 20 weeks.
**Option C:** This is incorrect as surfactant appears in amniotic fluid at a later gestational age than 32 weeks.

**Clinical Pearl / High-Yield Fact**
The presence of surfactant in amniotic fluid is a strong predictor of fetal lung maturity, and its absence is associated with an increased risk of respiratory distress syndrome in preterm infants.

**Correct Answer:** C. 32 weeks.