**Core Concept**
The question is testing the student's knowledge of immunostimulants, which are substances that enhance the body's immune response to infections or diseases. Immunostimulants can be used to prevent or treat infections, and they can also be used to stimulate the immune system in patients with weakened immune systems.

**Why the Correct Answer is Right**
The correct answer is a drug that activates the immune system by increasing the production of white blood cells, such as T cells and B cells. This is achieved through the activation of immune cells, such as macrophages and dendritic cells, which present antigens to T cells, stimulating an immune response. The correct answer works by increasing the production of cytokines, such as interferon and interleukin-2, which are essential for the activation of immune cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This drug is actually an immunosuppressant, which means it reduces the body's immune response. It is used to prevent the rejection of transplanted organs and to treat autoimmune diseases.
* **Option B:** This drug is an antimicrobial agent, which means it kills or inhibits the growth of microorganisms. While it may have some immunomodulatory effects, its primary mechanism of action is not as an immunostimulant.
* **Option C:** This drug is an antiviral agent, which means it inhibits the replication of viruses. While it may have some immunostimulatory effects, its primary mechanism of action is not as an immunostimulant.

**Clinical Pearl / High-Yield Fact**
Immunostimulants can be used to prevent or treat infections, particularly in patients with weakened immune systems, such as those with HIV/AIDS or undergoing chemotherapy. Examples of immunostimulants include interferon-alpha and interleukin-2.

**Correct Answer:** C.