Step 1: Understand the term 'endemic.'
Endemic species are those that are found only in a specific geographic area and are not found naturally elsewhere.
Step 2: Review the options.
- Natural refers to organisms that naturally exist in an environment, but this term does not refer to the population restricted to a particular area.
- Endemic refers to species that are native to and restricted to a specific area.
- Alien species are those that are introduced to an area outside their natural range.
- Pandemic refers to diseases that spread across countries or continents, not the population.
Step 3: Conclude.
Endemic species are confined to a particular area, making option (B) the correct answer.
Final Answer:
\[
\boxed{\text{Endemic}}
\]