the plasma membrane of the eukaryotic host the nuclear membrane of the eukaryotic host the ribosomes of the engulfed prokaryotic cell the lysosomes of the engulfed prokaryotic cell
I gave my students this multiple choice question: According to the endosymbiotic theory, double-membrane mitochondria originated as a result of some larger eukaryotic cells with flexible membranes engulfing smaller prokaryotic cells. Which of these explains where the outer layer of the double membrane of the mitochondria originated?

Only one answer is correct among these 4 choices:
- the plasma membrane of the eukaryotic host
- the nuclear membrane of the eukaryotic host
- the ribosomes of the engulfed prokaryotic cell
- the lysosomes of the engulfed prokaryotic cell

Could you tell me which one is correct?

the plasma membrane of the eukaryotic host