According to the endosymbiotic theory of the origin of eukaryotic cells, how did mitochondria originate?

From engulfed, originally free-living bacteria
When a protoeukaryote engaged in a symbiotic relationship with a protocell by secondary endosymbiosis
From the nuclear envelope folding outward and forming mitochondrial membranes
From infoldings of the plasma membrane, coupled with mutations of genes for proteins in energy-transfer reactions