Flora is the term used to denote plants of a particular region or period. This term is commonly used in the field of botany and ecology to refer to the collective plant life found in a specific geographical area or during a specific time period. It includes all types of plants, such as trees, shrubs, herbs, grasses, ferns, mosses, and algae.

Flora plays a crucial role in the ecosystem as it provides food, shelter, and habitat for various organisms, including animals, insects, and microorganisms. It also helps in maintaining the balance of the ecosystem by contributing to the oxygen and carbon dioxide cycle.

Flora can vary significantly from one region to another due to differences in climate, soil composition, and other environmental factors. Each region has its own unique set of plant species that are well-adapted to the local conditions. For example, tropical rainforests have a diverse and lush flora, while deserts have a sparse and drought-tolerant flora.

Similarly, flora can also change over time due to factors such as climate change, geological events, and human activities. The study of historical flora, known as paleoflora, helps scientists understand the evolution of plant life over millions of years. By analyzing fossilized plant remains, scientists can reconstruct the types of plants that existed in different periods of Earth's history.

In the context of the given options, ferns (option A) are a specific type of plant and do not encompass all the plants of a region or period. Fauna (option C) refers to the animal life of a particular region or period and does not include plants. Therefore, the correct answer is option B - flora.