Endosymbiont Theory Is Well-Accepted for One Evolutionary Process but Controversial

Multiple Choice

Endosymbiont theory is well-accepted for one evolutionary process but controversial for explaining another.What are these two processes, respectively?

A) eukaryotic nucleus development and horizontal gene transfer via gene fusion
B) horizontal gene transfer via gene fusion and eukaryotic nucleus development
C) horizontal gene transfer via gene fusion and mitochondria formation