In the origin of the eukaryotes, endosymbiosis:
a. describes the engulfing of a photosynthetic bacterium that ultimately gave rise to mitochondriab. most likely began with the endosymbiont entering the host cell through the endomembrane systemc. most likely began with a heterotrophic endosymbiont entering the cell as undigested prey or as an internal parasitee. is thought to explain the origin of the nucleus

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

Explanation:

The endosymbiosis theory  is based on the facts that Mitochondria and chloroplast consists of certain  similar ribosomes in size the the set of ribisomes found in bacteria. Thus when measure these ribosomes  were measured to be 70s compare to 80s found in the cytoplasm. In addition these organelles also contain special  circular DNA different from the one in  the normal cells nucleus.

Therefore it was theorized that chloroplasts and mitochondria must have  existed as  bacteria, which now live in a bigger hosts(animals and plants).

Thus this justified the answer;most likely began with a heterotrophic endosymbiont entering the cell as undigested prey or as an internal parasite,

b ti isnfornopgnoernfrof

i think that it is c, d, and e, hope this