Step 1: Name the four membranes with a short memory aid.
Think of the letters A, C, A, Y: amnion, chorion, allantois, and yolk sac. That is four separate extra-embryonic membranes formed during human development.
Step 2: Give each one a one-line job.
Amnion holds the fluid that cushions the embryo. Chorion forms the outer layer that becomes part of the placenta. Allantois helps with early blood vessel formation and waste storage. Yolk sac gives early nutrition before the placenta takes over.
Step 3: Confirm none of these four overlaps or repeats.
Each of the four membranes has its own separate structure and job, so they are counted as four distinct membranes, not fewer.
Step 4: Rule out the other counts.
Three, two, and one are all undercounts, since they leave out one or more of amnion, chorion, allantois, or yolk sac.
Final Answer:
Four foetal membranes form during human embryo development, option (2).