I think it would be "Skunks’ spray is a powerful defense that is difficult to avoid, but skunks do warn before spraying."
Explanation:
Because the other sentences are either talking about the things that get mentioned in the paragraph or only a part of the paragraph. But, the "Skunks' spray is a powerful defense that is difficult to avoid, but skunks do warn before spraying", sums up the whole paragraph. :)
