Assertion: Cnidarians exhibit cellular level  of organisation and are diploblastic.

Reason: Cnidarians have a central gastro-vascular cavity with a single opening, mouth on hypostome.

Assertion is incorrect but reason is correct.

The correct answer is Option (4) -Assertion is incorrect but reason is correct.

Cnidoblasts or cnidocytes (which contain the stinging capsules or nematocysts) present on the tentacles and the body. Cnidoblasts are used for anchorage, defense and for the capture of prey . Cnidarians exhibit tissue level of organisation and are diploblastic. They have a central gastro-vascular cavity with a single opening, mouth on hypostome. Digestion is extracellular and intracellular. Some of the cnidarians, e.g., corals have a skeleton composed of calcium carbonate.

Cnidarians exhibit two basic body forms called polyp and medusa . The former is a sessile and cylindrical form like Hydra, Adamsia, etc. whereas, the latter is umbrella-shaped and free-swimming like Aurelia or jelly fish.

Those cnidarians which exist in both forms exhibit alternation of generation (Metagenesis), i.e., polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).