The endosymbiotic theory proposes that mitochondria in eukaryotic cells arose from

The endosymbiotic theory proposes that mitochondria in eukaryotic cells arose from a prokaryote that lived in symbiosis with a eukaryotic cell.

Explanation:

The endosymbiotic theory was proposed by Lynn Margulis in 1981 and admits that some organelles (mitochondria and chloroplasts) in eukaryotic cells arose thanks to a symbiotic association.  It is believed that mitochondria and chloroplasts are descendants of prokaryotic organisms that were captured and adopted by some cell, thus living in symbiosis.

a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.