What is a uniquely human challenge? what part of the body is reponsible for this?

I believe its control of emotions. Humans have the most complex emotional responses than any other species on earth. Therefore the endeavour to make emotions and reason congruous are a uniquely humans challenge. The part of the humans responsible for emotions and reason is the limbic system of the brain. It constitutes the hippocampus, amygdala, limbic cortex and septal area.

the answer to your question is sperm

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."