Which feature is found most often in prokaryotic cells?
a) plasmid
b) flagella
c) cytoplasm
d) cell membrane

If we're saying found more often in prokaryotic versus eukaryotic cells, the answer would probably be flagella. Eukaryotic and prokaryotic cells all have cytoplasm and cell membranes at the very least, and its more common in prokaryotic but eukaryotic cells could also have plasmids. Flagella could be occurrent in eukaryotic cells as well, particular in unicellular organisms like protists and even sperm cells, but that feature is probably more common with prokaryotic cells out of all the options in the list. 

I would go with the plasmid because the covering is made of it

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