The theory of endosymbiosis suggests that the ancestors of mitochondria were

The theory of endosymbiosis suggests that the ancestors of mitochondria were  once free-living heterotrophic prokaryotic cells.

Explanation:

The endosymbiosis theory states that eukaryotes originated from an endosymbiosis of prokaryotic precursors. Accordingly, chemo- and phototrophic bacteria have been taken up by archaea, in which they have developed into cell organelles in their host cells, including mitochondria and plastids. However, there are also eukaryotes without such organelles that do neither cellular respiration nor photosynthesis, whereby it is assumed that these cell components have subsequently been lost.