This is a characteristic feature of coelenterata

The correct option is Gastrovascular cavity.

Coelenterata (also known as Cnidaria) is a phylum of aquatic, mostly marine, radially symmetrical animals. Let us understand why the presence of a gastrovascular cavity is a characteristic feature of this phylum by analyzing the options:
1. Gastrovascular cavity (Coelenteron): This is the central body cavity of coelenterates, which has a single opening called the mouth (surrounded by tentacles) that serves for both ingestion of food and egestion of waste. This cavity is responsible for both digestion and the distribution of nutrients throughout the body, hence the name gastrovascular cavity. The name 'Coelenterata' itself is derived from this characteristic cavity (coelos meaning hollow and enteron meaning gut).
2. Polyp: While the polyp is a body form found in coelenterates, it is not universal or unique as a defining characteristic of all members, because many coelenterates also exhibit the medusa form or alternate between polyp and medusa stages (metagenesis).
3. Presence of tentacles around mouth: Although tentacles are present around the mouth, they are also found in several other invertebrate groups (like some mollusks or ctenophores), making it a less exclusive defining feature than the coelenteron.
4. All are marine: While most coelenterates are marine, not all are; freshwater species like Hydra exist, making this statement incorrect.

Therefore, the presence of a gastrovascular cavity is a primary defining characteristic of the phylum Coelenterata.