**Core Concept**
The question is testing the concept of immunogenicity, specifically the ability of a substance to stimulate both humoral (antibody-mediated) and cell-mediated immunity. This involves the activation of immune cells, such as T-cells and B-cells, and the production of antibodies.

**Why the Correct Answer is Right**
Keyhole limpet hemocyanin (KLH) is a highly immunogenic protein derived from the keyhole limpet, a type of marine snail. It is widely used as an immunological reagent due to its ability to stimulate a strong and consistent immune response. KLH is particularly effective at activating both T-cells and B-cells, leading to the production of antibodies and the activation of cell-mediated immune responses.

* KLH contains multiple epitopes, which are regions on the protein that are recognized by the immune system.
* The protein is highly stable and resistant to degradation, making it an ideal antigen for immunological studies.
* KLH has been shown to stimulate both humoral and cell-mediated immunity in a variety of experimental models.

**Why Each Wrong Option is Incorrect**
**Option A:** Tetanus toxoid is a potent antigen, but it is primarily used to stimulate humoral immunity and does not elicit a strong cell-mediated response.
**Option B:** Influenza virus is a complex pathogen that can stimulate both humoral and cell-mediated immunity, but it is not as potent an antigen as KLH.
**Option C:** Keyhole limpet lectin is a different protein from KLH and is not as widely used as an immunological reagent.

**Clinical Pearl / High-Yield Fact**
KLH is often used as a carrier protein to enhance the immunogenicity of smaller antigens, such as peptides or haptens. This is because the immune system recognizes KLH as a strong antigen and can respond to it more readily than to smaller, less immunogenic molecules.

**Correct Answer:** C. Keyhole limpet hemocyanin.