Cnidarians share a common body plan that takes two forms. The free-floating form is called a medusa, whereas the sessile form is called a:

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The question specifically mentions that there are two forms that cnidarians take, and provides the names of those forms. The medusa is described as free-floating, while the other form is described as sessile. The only option that fits this description is A, polyp, which is the sessile form of a cnidarian. None of the other options accurately describe the two forms of cnidarians mentioned in the question.

Cnidarians Share a Common Body Plan That Takes Two Forms