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

Endosymbiosis is most likely began with a heterotrophic endosymbiont entering the cell as undigested prey or as an internal parasite

Explanation:

Endosymbiosis can be an organism which lives in the body of another organism which may or may not be can be mutual.

Endosymbiosis explains the evolution of eukaryotes from prokaryotes.

Mitochondria having its own DNA is an example that it evolved from a prokaryotic organism as bacteria which inhabited as endosymbiont in the body cells and eventually became an organelle.  This defines the endosymbiosis very well.

Other organelles as chloroplast and plastids are also regarded to have  evolution by endosymbionts.

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explanation:

because i added numbers