Which one of the following groups of animal reproduces only by sexual means?

Options

A. Cnidaria and Platyhelminthes
B. Porifera
C. Protozoa
D. Ctenophora

Explanation

To determine which group of animals reproduces only by sexual means, we can analyze the provided options step by step:

### Step 1: Analyze the Options
We have several groups of animals listed as options: Naderia, Platyhelminthus, Porifera, Protozoa, and Tenophora. We need to evaluate each group to see if they reproduce only by sexual means.

### Step 2: Evaluate Naderia
- Naderia includes organisms like jellyfish and corals.
- They can reproduce both sexually (in the medusa form) and asexually (in the polyp form).
- Conclusion: Naderia does not reproduce only by sexual means.

### Step 3: Evaluate Platyhelminthus
- Platyhelminthus includes flatworms, such as Planaria.
- They can reproduce asexually through regeneration (e.g., cutting a Planaria into pieces).
- Conclusion: Platyhelminthus does not reproduce only by sexual means.

### Step 4: Evaluate Porifera
- Porifera includes sponges.
- They can reproduce sexually (producing gametes) and asexually (through fragmentation).
- Conclusion: Porifera does not reproduce only by sexual means.

### Step 5: Evaluate Protozoa
- Protozoa are unicellular organisms, often classified under the kingdom Protista.
- They primarily reproduce asexually through processes like binary fission.
- Conclusion: Protozoa do not reproduce only by sexual means.

### Step 6: Evaluate Tenophora
- Tenophora includes organisms like sea walnuts and comb jellies.
- They reproduce exclusively through sexual means, with no asexual reproduction observed.
- Conclusion: Tenophora reproduces only by sexual means.

### Final Answer
Based on the evaluations, the group of animals that reproduces only by sexual means is **Tenophora**.

---