Looking into a microscope, you notice cells swimming, propelled by a long tail. What cell structure must these human cells have in order to be mobile:

I believe the answer is C. flagella

Flagella is a hair like organelle that capable of moving. Flagella would be look like a tail that could be moved to propel the cell forward. The only human cells that have flagella would be sperm as it need to move some distance in order to fertilize the ovum in uterus. Low motility of the sperm could cause infertility.