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 (not in options here but for context) originated as free - living prokaryotes that were engulfed by a host cell. Mitochondria are thought to have been aerobic bacteria. This theory helps explain the origin of mitochondria. Cell membranes, nuclei, and ribosomes are not explained by the endosymbiotic theory.

Answer:

mitochondria