A. eukaryotic organisms engulfed prokaryotic organisms, eventually developing into animals and plants. b. larger prokaryotes engulfed smaller prokaryotes, eventually developing into eukaryotic organisms. c. smaller prokaryotes engulfed larger prokaryotes, eventually developing into eukaryotic organisms. d. larger eukaryotes engulfed smaller eukaryotes, eventually developing into animals and plants.

There is no question in this, but the correct answer can be deduced from the choices. This might be asking for the true statement between the choices. 

The correct answer is choice A, "eukaryotic organisms engulfed prokaryotic organisms, eventually developing into animals and plants". This is in accordance to the theory of endosymbiosis wherein a primitive eukaryotic organism engulf prokaryotic organisms that can do photosynthesis (chloroplast) and cellular respiration (mitochondria); and these will eventually develop into higher order organisms such as plants and animals. 

Other choices are wrong because in choice B and C and D, these are not how eukaryotic organisms emerged but from endosymbiosis. 

Option B is the only option that is True.
Options A, C and D are False.

The endosymbiosis theory, which seeks to explain the process by which prokaryotes gave rise to the first eukaryotic cells, states that the evolution of eukaryotic cells was as a result of a larger prokaryotic cell engulfing a smaller prokaryotic cell without digesting it.

The higher you go up the less oxygen there is