Question

the endosymbiotic theory helps to explain the origin of which structures? mitochondria ribosomes nuclei cell membranes

Explanation:

The endosymbiotic theory proposes that mitochondria (and chloroplasts) originated from free - living prokaryotes that were engulfed by a larger cell. Mitochondria have their own DNA, ribosomes, and double membranes, which are characteristics that support the idea of an endosymbiotic origin. Ribosomes are found in all cells (prokaryotic and eukaryotic) and have a more universal and simpler evolutionary story not related to endosymbiosis. Nuclei have a different evolutionary origin story, likely through membrane - infolding and compartmentalization. Cell membranes are a basic feature of all cells and their origin is more related to the formation of the first cells from lipid bilayers and not through endosymbiosis.

Answer:

mitochondria