Reproductive anatomy (mammals): Identify what tissues form the placenta. Full question: Placenta is the structure formed by the apposition/integration of foetal and maternal tissues—specifically, which combination? Choose the correct option.

Assumption and clarification
The original prompt reads ‘‘Placenta is the structure formed’’ (missing ‘‘by’’). Interpreting the intended question as ‘‘Placenta is the structure formed by what?’’

Concept
In eutherian mammals, the placenta develops from foetal chorionic villi and the maternal endometrium (specifically the decidua basalis). It mediates nutrient, gas, and waste exchange between mother and foetus.

Why others are incorrect
Amnion/allantois and yolk sac are extraembryonic membranes with supporting roles; the umbilical cord connects foetus to the placenta but does not form it; direct mixing of foetal liver with maternal blood does not occur.

Final Answer
Chorionic villi of the foetus and the maternal uterine endometrium (decidua basalis).