Comprehensive and Detailed Explanation From NCC-Aligned Physiologic Sources:

Oxygen transfer occurs at the maternal–fetal interface within the intervillous space, where:

Maternal blood from the spiral arteries bathes the chorionic villi

Diffusion occurs between maternal blood and fetal capillary beds

Oxygen then travels through fetal circulation via the umbilical vein

Thus:

Intervillous space = site of gas exchange

Spiral arteries = deliver maternal blood to that space

Umbilical vein = fetal vessel carrying oxygenated blood after exchange has occurred

Correct answer: A. Intervillous space