### Step-by-Step Solution:
1. **Understanding Lichens**:
- Lichens are unique organisms that result from a symbiotic relationship between two different types of living organisms: algae and fungi.
2. **Identifying the Options**:
- The options provided are:
- Parasites
- Saprophytes
- Symbionts
- Chemotrophs
3. **Analyzing Each Option**:
- **Parasites**:
- Parasites are organisms that live on or in a host and derive nutrients at the host's expense. They often cause harm to the host. Since lichens do not harm their partners but rather benefit from the relationship, this option is incorrect.
- **Saprophytes**:
- Saprophytes are organisms that obtain nutrients from dead and decaying organic matter. Lichens do not fit this definition as they do not solely rely on dead material for sustenance. Therefore, this option is also incorrect.
- **Symbionts**:
- Symbionts are organisms that engage in a symbiotic relationship, where both parties benefit from the association. In the case of lichens, the fungi provide structure and protection, while the algae conduct photosynthesis to produce food. This mutual benefit makes this option correct.
- **Chemotrophs**:
- Chemotrophs are organisms that obtain energy from chemical compounds, either organic or inorganic. Lichens do not primarily derive their energy this way, making this option incorrect.
4. **Conclusion**:
- Based on the analysis, the correct answer is that lichens are **symbionts** (Option 3).
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