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 in plant cells) originated from free - living prokaryotes that were engulfed by a larger cell. Ribosomes are made of rRNA and proteins and are synthesized in the nucleolus. Nuclei have their own theories of origin not related to endosymbiosis. Cell membranes are a basic feature of all cells and their origin is not explained by the endosymbiotic theory.

Answer:

mitochondria