**Lichens**

Lichens are the result of a mutualistic relationship between algae or cyanobacteria (a type of bacteria that can perform photosynthesis) and fungi. This unique partnership is called a symbiotic relationship, where both organisms benefit from living together. Lichens are often found growing on rocks, tree bark, and other surfaces in diverse environments, including forests, deserts, and even the Arctic tundra.

Below is a detailed explanation of why the correct answer is option 'D' - Lichens:

**Symbiosis:**
Symbiosis refers to a close and long-term interaction between two or more different species, where they live together and have a mutually beneficial relationship. In the case of lichens, the partnership between algae (or cyanobacteria) and fungi is a prime example of symbiosis.

**Algae and Fungus:**
The two main components of lichens are algae (or cyanobacteria) and fungi. Algae are photosynthetic organisms that use sunlight to produce energy-rich organic compounds through the process of photosynthesis. Fungi, on the other hand, are non-photosynthetic organisms that obtain nutrients by decomposing organic matter.

**Mutualistic Relationship:**
When algae and fungi come together to form lichens, they create a mutually beneficial relationship. The algae or cyanobacteria provide food through photosynthesis, producing sugars and other organic compounds that the fungi can utilize for energy. In return, the fungi provide a protective environment for the algae or cyanobacteria, as well as absorb and retain water and nutrients from the environment.

**Structure of Lichens:**
Lichens have a unique structure that allows for this mutualistic relationship. The algae or cyanobacteria live inside the fungal tissue, forming a layer called the photobiont. The fungi, known as the mycobiont, surround and protect the photobiont. This structure enables the exchange of nutrients and provides a suitable habitat for both organisms.

**Adaptations and Benefits:**
Lichens have evolved various adaptations that enable them to survive in harsh environments. They can tolerate extreme temperatures, high levels of radiation, and low nutrient availability. The algae or cyanobacteria contribute to this resilience by producing energy through photosynthesis, while the fungi provide protection and help absorb moisture.

**Conclusion:**
In conclusion, the correct answer to the question is option 'D' - Lichens. Lichens represent a symbiotic relationship between algae (or cyanobacteria) and fungi, where both organisms benefit from living together. This unique partnership allows lichens to thrive in diverse environments and contribute to ecological processes such as soil formation and nitrogen fixation.