In order to explain how the first eukaryotic cells may have evolved, scientists proposed the a. theory of solar nebula. b. theory of spontaneous generation. c. theory of endosymbiosis. d. theory of autotropic evolution.

The correct answer is option C, that is, the theory of endosymbiosis.  

The endosymbiotic theory deals with the origins of chloroplasts and mitochondria, the two eukaryotic organelles possessing bacterial features. Mitochondria and chloroplasts are considered to have generated from symbiotic bacteria, mainly the alpha-proteobacteria and cyanobacteria.  

According to the endosymbiotic theory the eukaryotes developed via a process whereby distinct kinds of free-living prokaryotes became integrated within the bigger prokaryotic cells and finally resulting into chloroplasts, mitochondria, and possibly other organelles.  

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