Which of the following was an essential step in the evolution of eukaryotic cells?

All of these are necessary to the evolution of eukaryotics. They need a flexible surface to survive by themselves (unicellularly), they need a cytoskeleton which is important to locomotion and cell division. They need a nuclear envelope to enclose their nuclei. And the endosymbiotic acquisition of certain organelles is what allowed them to develop alot, for instance taking up mitochondria, and taking up cyanobacteria (first photosynthesizers) to create chloroplasts. All of these were necessary.