The endosymbiotic theory states that precursor eukaryotic cells acquired cilia from which type of ancestor?

The endosymbiotic theory proposes that eukaryotic cells originate from symbiotic relationships between different species of prokaryotic organisms. According to this theory, certain organelles in eukaryotic cells, such as mitochondria and chloroplasts, are derived from free-living bacteria that were engulfed by an ancestral eukaryotic cell.

In the context of acquiring cilia, flagella are often associated with single-celled eukaryotes like protozoa, which are proto-eukaryotic organisms that may have shared a common ancestor with later eukaryotes. Flagellated protozoans, which possess flagella as motility structures, provide insight into the origin of cilia, as they represent a lineage that would have contributed to the cellular complexity observed in the evolution of eukaryotic cells. The presence of flagella in older, simpler cells indicates that, over time, these structures could have evolved into cilia for locomotion and feeding in more complex organisms.

The spiral ancestor option lacks the necessary context, as it does not clearly relate to any known group of organisms recognized in the endosymbiotic theory or the evolution of cilia. Recognizing that flagellated protozoa directly relate