**Core Concept**
The question pertains to carbohydrate antigens, specifically focusing on tumor-associated antigens that are often used as targets in cancer immunotherapy. Carbohydrate antigens, such as mucin-type O-glycans, are expressed on the surface of various cancer cells and can induce an immune response.

**Why the Correct Answer is Right**
Carbohydrate antigens, like Tn antigen, are indeed expressed on the surface of cancer cells and can be targets for immunotherapy. The Tn antigen is a mucin-associated O-glycan that is often overexpressed in various cancers, making it a potential target for cancer vaccines. Similarly, the TF (Thomsen-Friedenreich) antigen is another mucin-associated O-glycan that is expressed on the surface of cancer cells and can be targeted by the immune system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be incorrect if it contradicts the established knowledge on carbohydrate antigens in cancer immunotherapy.
**Option B:** This option might be incorrect if it presents a carbohydrate antigen that is not typically associated with cancer or is not a valid target for immunotherapy.
**Option C:** This option might be incorrect if it inaccurately describes the expression or function of a carbohydrate antigen in the context of cancer.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that carbohydrate antigens like Tn and TF are often overexpressed on the surface of cancer cells, making them potential targets for immunotherapy. This understanding is crucial for developing effective cancer vaccines and therapies.

**Correct Answer: A.**