Cockroach has separate male and female bodies, hence it is unisexual. Rest all the animals listed are hermaphrodites in which male and female gametes are produced in the same animal body.
So, the correct answer is option 'B'.

**Unisexual Organism**

A unisexual organism, also known as a dioecious organism, refers to a species in which individuals are either male or female and have separate reproductive organs. In other words, unisexual organisms have distinct male and female individuals within the same species. These individuals are specialized for either producing sperm (male) or eggs (female).

**Examples of Unisexual Organisms**

Among the given options, the correct example of a unisexual organism is the **cockroach (option B)**. Cockroaches are dioecious organisms, meaning they have separate male and female individuals within the species. Male cockroaches have specialized reproductive organs for producing sperm, while female cockroaches have specialized organs for producing eggs.

Let's briefly discuss the other options and why they are not examples of unisexual organisms:

- **Earthworm (option A):** Earthworms are hermaphroditic organisms, meaning they possess both male and female reproductive organs. They can produce both sperm and eggs, and individuals can exchange sperm during mating.

- **Tapeworm (option C):** Tapeworms are also hermaphroditic organisms. They have both male and female reproductive structures within their bodies and are capable of self-fertilization.

- **Leech (option D):** Leeches are hermaphroditic organisms as well. They possess both male and female reproductive organs and can self-fertilize.

Therefore, the correct answer is option B, the cockroach, as it is the only example among the given options that exhibits separate male and female individuals, making it a unisexual organism.