Organ of defense in protozoans is

The correct answer is trichocysts.

Let's understand the organs of defense and other structures mentioned in the options to see why trichocysts is the correct choice:
1. Trichocysts: These are specialized spindle-shaped organelles found in the ectoplasm of certain ciliates, such as Paramecium (a protozoan). When the protozoan is threatened by a predator or stimulated mechanically/chemically, these organelles discharge long, sticky, thread-like structures. They serve as an organ of defense, allowing the organism to deter predators, and can also help in anchoring the organism or capturing prey.
2. Statocysts: These are sensory receptors responsible for balance and orientation (gravitaxis) in various multicellular invertebrates, such as cnidarians, mollusks, and crustaceans, but are not found in unicellular protozoans.
3. Otocysts: Similar to statocysts, these are auditory or equilibrating vesicles found in multicellular organisms (such as certain mollusks and invertebrates) and are not present in protozoans.
4. Nematocysts: These are specialized stinging cells (cnidocytes) containing a coiled thread that can inject toxin. They are characteristic of the phylum Cnidaria (coelenterates), which are multicellular animals, rather than unicellular protozoans.

Therefore, among the options provided, the organ of defense specifically found in protozoans is the trichocyst.