Nthe origin of the eukaryotes, endosymbiosis in the origin of the eukaryotes, endosymbiosis most likely began with a heterotrophic endosymbiont entering the cell as undigested prey or as an internal parasite. most likely began with the endosymbiont entering the host cell through the endomembrane system. describes the engulfing of a photosynthetic bacterium that ultimately gave rise to mitochondria. is thought to explain the origin of the nucleus.

These are the choices for this question:

(1) Endosymbiosis most likely began with a heterotrophic endosymbiont entering the cell as undigested prey or as an internal parasite.
(2) Most likely began with the endosymbiont entering the host cell through the endomembrane system
(3) Describes the engulfing of a photosynthetic bacterium that ultimately gave rise to mitochondria.
(4) Is thought to explain the origin of the nucleus.

The correct answer is "endosymbiosis most likely began with a heterotrophic endosymbiont entering the cell as undigested prey or as an internal parasite". Endosymbiosis is described to be the origin of eukaryotic cell by a cell engulfing two prokaryotic organisms, one capable of cellular respiration and one capable of photosynthesis. The endosymbiont did not enter the cell through the endomembrane system as lysosomal/peroxisomal enzymes may kill these organisms instantly. 

Choice (3) is wrong as the photosynthetic bacterium gave rise to the chloroplast, not the mitochondria.