Exoskeleton is a characteristic feature of

Step 1: Analyze the question The question asks which organism has an exoskeleton, which is a hard external covering that supports and protects the body.

Step 2: Explain the concept An exoskeleton is a rigid outer covering. In arthropods (insects, crustaceans, arachnids, centipedes, millipedes) it is made of chitin. It protects the animal, prevents water loss, and gives support and attachment for muscles. Arthropods must shed it (moult or ecdysis) in order to grow.

Step 3: Examine each option

• A. Amoeba: a single-celled protozoan with only a flexible cell membrane and no exoskeleton.
• B. Earthworm: an annelid with a soft, moist body. It has a hydrostatic skeleton (fluid-filled coelom), not an exoskeleton.
• C. Hydra: a cnidarian with a soft, sac-like body and no hard covering.
• D. Millipede: an arthropod (class Diplopoda) with a hard, chitinous exoskeleton made of segments, and two pairs of legs per segment.
• E. Tapeworm: a flatworm and parasite with a soft body covered by a protective cuticle (tegument). This is not a true exoskeleton.

Step 4: Answer Only the millipede is an arthropod, so it is the one with an exoskeleton. The correct option is D.

Tip: Think "Arthropod = Armour." If the animal has jointed legs and a segmented body, such as an insect, crab, spider, centipede or millipede, it has a chitinous exoskeleton. Soft-bodied animals (worms, hydra, amoeba) do not.