Select all the examples of cnidarians that only exist as sessile polyps.

The question you asked doesn't provide a list of cnidarians to select from. However, I can tell you that some examples of cnidarians that only exist as sessile polyps include the Hydra and the sea anemone. These organisms do not have a medusa stage in their life cycle, which is the mobile stage that other cnidarians, like jellyfish, go through.

The question seems to be incomplete as there are no options provided to select from. However, to answer your question, some examples of cnidarians that only exist as sessile polyps include sea anemones and coral.