The right answer is a surface glycoprotein.
The antigens of the non-self correspond to any substance foreign to the organism, identified as such by the immune system of our organism which produces an energetic defense. Antigens are usually proteins (more specifically glycoproteins) contained in cells or foreign bodies (red blood cells transfused, organs transplanted, bacteria, viruses), or present in the environment (pollen, mite droppings, cat hair , etc.).
Our body has its own antigens that allow it to have a "specific identity card" and will be recognized as foreign by other organizations. They are called "self antigens".