The correct answer is option 'C', Fauna.

Fauna refers to the animal species of a particular region or period. It is derived from the Latin word "Fauna" which means the goddess of animals. The term is used to describe the collective animal life of a specific geographical region or a particular time period.

Explanation:

1. Definition of Fauna:
- Fauna refers to the animals that inhabit a particular region or period.
- It includes all the animal species, both vertebrates (such as mammals, birds, reptiles, amphibians, and fish) and invertebrates (such as insects, spiders, crustaceans, mollusks, and worms).
- Fauna is a term used in the field of biology and ecology to study and describe the animal life in different ecosystems.

2. Difference between Flora and Fauna:
- Flora refers to the plant species of a particular region or period, while fauna refers to the animal species.
- Flora and fauna are collectively used to describe the biodiversity of an area.

3. Importance of Fauna in Ecology:
- Fauna plays a crucial role in maintaining the ecological balance of an ecosystem.
- Each species in a fauna has its own ecological niche and contributes to the overall functioning of the ecosystem.
- The presence or absence of certain animal species can have a cascading effect on the entire food web and ecosystem dynamics.

4. Fauna of a Specific Region or Period:
- The fauna of a specific region is determined by various factors such as climate, geography, and habitat availability.
- Different regions have their own unique fauna, characterized by the presence of certain species and the absence of others.
- Similarly, the fauna of a particular period in Earth's history can be studied through the fossil record, providing insights into the past biodiversity and evolution of animals.

In conclusion, the term "fauna" is specifically used to denote the animal species of a particular region or period. It is an essential concept in biology and ecology, helping us understand the diversity and dynamics of animal life in different ecosystems.