**Core Concept**
Rituximab is a monoclonal antibody that targets a specific protein on the surface of B cells, leading to their depletion and subsequent immunosuppression. This mechanism is crucial in the treatment of various B-cell malignancies and autoimmune diseases.

**Why the Correct Answer is Right**
Rituximab specifically binds to the CD20 antigen, which is expressed on the surface of mature B cells (from the pre-B cell stage to the plasma cell stage) but not on hematopoietic stem cells, pro-B cells, or plasma cells. This selective binding leads to the activation of complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC), resulting in the destruction of B cells. Rituximab's efficacy in treating conditions like non-Hodgkin's lymphoma and rheumatoid arthritis is largely due to its ability to deplete B cells and modulate the immune response.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because Rituximab does not target CD4+ T cells, which are a different subset of immune cells.
**Option B:** Incorrect because Rituximab does not target the CD52 antigen, which is actually targeted by Alemtuzumab in the treatment of chronic lymphocytic leukemia (CLL).
**Option C:** Incorrect because Rituximab does not target the CD25 antigen, which is actually targeted by Basiliximab in the prevention of organ transplant rejection.

**Clinical Pearl / High-Yield Fact**
Rituximab's mechanism of action is unique among monoclonal antibodies, making it a valuable treatment option for conditions characterized by B-cell overactivity. It's essential to remember that Rituximab's efficacy is largely dependent on its ability to selectively target CD20+ B cells, minimizing its impact on other immune cells.

**Correct Answer: D. CD20**