What must be true if unicellular organisms evolved into organisms in the domain eukarya?

The choices are:
A. Absorbing other cells gave a competitive advantage.
B. The formation of amino acids gave a survival advantage.
C. Being unicellular gave a survival advantage.
D. Having genetic material gave a survival advantage.

The right answer is: A
Eukaryotes (or Eukaryota) are a group of all organisms, unicellular or multicellular, characterized by the presence of a nucleus and usually mitochondria in their cells. He opposes the domains of Eubacteria and Archaea.

"Absorbing other cells gave a competitive advantage" is referring to the endosymbiosis of the mitochondria.
The mitochondrion is the result of the endosymbiosis of an alpha-proteobacterium by a primitive eukaryotic cell.
The existence of genes of endosymbiosis (transferred to the nucleus of the host cell and integrated into the genome of the latter) or their vestiges (remaining in the nucleus while the organelles themselves are lost or degenerated) reveals that the ancestors of eukaryotes devoid of mitochondria once contained such organelles.

When a virus infects a host cell, it uses the cell's machinery to make copies of its own genetic material in a process called replication. finally, during the process of self-assembly, that protein shell and the copies of the virus's genetic material are assembled into new viruses.