The characteristic larva in coelenterates is

The correct option is Planula.

Coelenterates (also known as Cnidarians), which include organisms like jellyfish, sea anemones, and corals, exhibit a life cycle that often includes a larval stage. The characteristic larval form in this phylum is the planula larva.

Let us break down why this is the correct answer and examine the other options for clarity:
1. Planula: This is a free-swimming, flattened, ciliated, and bilaterally symmetrical larval stage typical of many coelenterates. It develops from the fertilized egg (zygote) and swims in the water column before settling onto a substrate to develop into a polyp.
2. Oncosphere: This is the larval stage of tapeworms (Phylum Platyhelminthes), characterized by having six hooks.
3. Rhabditiform: This is the early larval stage of certain parasitic nematodes (Phylum Nematoda), such as hookworms.
4. Cysticercus: Also known as a bladder worm, this is a larval stage of tapeworms (Phylum Platyhelminthes) that is encysted in the tissues of a host.

Therefore, the planula is uniquely associated with coelenterates.