Antigens are substances that can trigger an immune response in the body. They can be present in various locations, but the correct answer to the question is option 'D', which states that antigens are present on the cell surface.

Explanation:
Antigens are usually proteins or large polysaccharides that are foreign to the body or altered versions of normal body proteins. When these antigens are detected by the immune system, they can be recognized as "non-self" and trigger an immune response to eliminate them.

Antigens can be present in different locations in the body, including:

1. On the cell surface: Many antigens are present on the surface of cells, where they can be detected by immune cells such as T cells and B cells. These cell surface antigens can be specific to certain types of cells or present on all cells in the body.

2. In the extracellular space: Some antigens can be released into the extracellular space, where they can be detected by antibodies produced by B cells. These antigens can come from pathogens such as bacteria or viruses, or from damaged tissue.

3. Inside the cell: Antigens can also be present inside the cell, either in the cytoplasm or inside organelles such as the nucleus or mitochondria. These antigens can be released from cells when they die or become damaged, and can also be presented on the cell surface by specialized cells called antigen-presenting cells.

In summary, antigens can be present in various locations in the body, but they are most commonly found on the surface of cells. These cell surface antigens can be detected by immune cells and trigger an immune response to eliminate the foreign or altered cells.