The Eukaryotic cells can also have 70s ribosomes think of organelles such as mitochondria and chloroplasts which do have 70s ribosomes which are similar to what found in a bacterium.
since, WBC’s have mitochondria the presence of 70s ribosomes is thus possible.
Also, all the other given structures in the answer options are membrane bound and as we know prokaryotes don’t have membrane bound organelles. Thus, all we are left with is answer option c.
Hope it helps:)