Most eukaryotic cells contain mitochondria, but a few single-celled eukaryotic organisms found in oxygen-free environments do not. Which of the observations led biologists to reject the hypothesis that these mitochondria-free eukaryotes evolved before the endosymbiotic event that established mitochondria in other eukaryotes?

Their nuclear genome contained relics of mitochondrial genes

Explanation:

Mitochondrion is a double membrane bound organelle that is found in most eukaryotic organism whose main function is to produce or generate energy. The mitochondria has DNA thus it is passed exclusively from mother to offspring.

It is hypothesized that it exists inside the cell after an endosymbiotic event

Since their nuclear genome contains relics of mitochondrial genes, biologists rejected the hypothesis that they evolved before the endosymbiotic event.