The answer is option 'B': Rare species

Explanation:
- Species that exist in small numbers are referred to as rare species.
- These species have a low population size and are not commonly found in a particular area or ecosystem.
- The rarity of a species can be determined by factors such as its population size, distribution, and habitat requirements.
- Rare species are often considered to be of conservation concern because their small populations make them more vulnerable to threats such as habitat loss, pollution, climate change, and overexploitation by humans.
- Conservation efforts are often focused on protecting and preserving rare species to prevent their decline or extinction.
- Rare species play an important role in maintaining biodiversity and ecosystem functioning.
- They may have unique ecological roles or provide important services such as pollination, seed dispersal, or pest control.
- Rare species can also have cultural or aesthetic value, as they may be considered symbols of biodiversity or have special significance to local communities.
- Conservation actions for rare species may include habitat protection, restoration, captive breeding programs, and public education and awareness campaigns.
- It is important to monitor and assess the status of rare species to inform conservation actions and ensure their long-term survival.
- Rare species are often listed as protected or endangered under national and international legislation, and their conservation status is evaluated by organizations such as the International Union for Conservation of Nature (IUCN).
- Overall, rare species are a valuable part of Earth's biodiversity and require special attention and conservation efforts to ensure their survival.