Radial symmetry is correctly matched with coelenterates.

Coelenterates, also known as Cnidarians, exhibit radial symmetry. Radial symmetry is a type of symmetry where the body parts are arranged around a central axis, similar to the spokes of a wheel. This arrangement allows the organism to be divided into equal halves in any plane passing through the central axis.

Coelenterates are a phylum of invertebrate animals that include jellyfish, sea anemones, and coral. They have a sac-like body plan with a central gastrovascular cavity. The body is organized around a central axis, with tentacles radiating outwards from the central region. These tentacles are armed with specialized stinging cells called cnidocytes, which they use for defense and prey capture.

Radial symmetry in coelenterates allows them to interact with their environment in all directions. They can capture prey from any direction, and their sensory structures, such as eyespots or statocysts, are distributed around the body. This symmetry is particularly advantageous for sessile or slow-moving organisms, as they can detect and respond to stimuli from any direction.

Other options in the question are incorrect matches:

- Coelomates are animals that possess a true coelom, a fluid-filled body cavity completely lined by mesoderm. Aschelminthes, also known as Nematodes, are pseudocoelomates, meaning they have a body cavity that is not completely lined by mesoderm. Therefore, option B is incorrect.

- Metamerism refers to the organization of the body into repeating segments or metameres. This characteristic is seen in annelids, such as earthworms, not molluscs. Therefore, option C is incorrect.

- Triploblastic refers to organisms that have three germ layers: ectoderm, mesoderm, and endoderm. Sponges, which belong to the phylum Porifera, are diploblastic, meaning they have only two germ layers. Therefore, option D is incorrect.