The correct statement according to the endosymbiotic theory is:
The nucleus was formed by the outward folding of the cell membrane at the same time that mitochondria was acquired.
The endosymbiotic theory suggests that certain organelles, such as mitochondria, originated as free-living prokaryotes that were engulfed by a host cell. The theory also posits that the nucleus formed through the invagination of the cell membrane, leading to the development of more complex eukaryotic cells.