All cnidarians share a few fundamental characteristics, but among the options provided, the correct answer is a) stinging cells. Cnidarians, such as jellyfish, corals, and sea anemones, possess specialized cells called cnidocytes, which contain stinging structures known as nematocysts. These stinging cells are used for capturing prey and defense.
While it is true that many cnidarians live in oceans, not all of them are exclusively marine; some can be found in freshwater environments. The medusa body form is also not common to all cnidarians, as they can also exist in polyp form, which is typical for organisms like corals and sea anemones.