**Core Concept**
Surface Immunoglobulin (sIg) is a key component of the B-cell receptor complex, allowing B-cells to recognize and respond to specific antigens. This surface-bound antibody plays a crucial role in initiating the immune response and facilitating B-cell activation, proliferation, and differentiation.

**Why the Correct Answer is Right**
B-cells express surface Immunoglobulin as part of their membrane-bound receptor complex, enabling them to recognize and bind to specific antigens. This interaction triggers B-cell activation, leading to the production of antibodies and the initiation of an immune response. The Immunoglobulin (Ig) molecule on the B-cell surface is composed of two heavy chains and two light chains, with the variable region of the heavy chain (VH) and the variable region of the light chain (VL) forming the antigen-binding site.

**Why Each Wrong Option is Incorrect**
**Option A:** T-cells do not express surface Immunoglobulin; instead, they possess T-cell receptors (TCRs) that recognize antigens presented by Major Histocompatibility Complex (MHC) molecules.
**Option C:** Natural Killer (NK) cells do not express surface Immunoglobulin; they rely on activating and inhibitory receptors to recognize and target infected cells or tumor cells.
**Option D:** Plasma cells are differentiated B-cells that have undergone terminal differentiation and have lost their surface Immunoglobulin expression, instead producing large amounts of soluble Immunoglobulin in the form of antibodies.

**Clinical Pearl / High-Yield Fact**
B-cells with surface-bound Immunoglobulin can be identified using flow cytometry or immunofluorescence staining, which can be useful in diagnosing B-cell malignancies or assessing B-cell function in autoimmune diseases.

✓ Correct Answer: B. B-cell