**Core Concept**
Daclizumab is a monoclonal antibody that targets the interleukin-2 receptor (IL-2R) on the surface of T lymphocytes. By binding to the alpha subunit of the IL-2R, daclizumab prevents the interaction between IL-2 and its receptor, thereby inhibiting T cell activation and proliferation.

**Why the Correct Answer is Right**
Daclizumab specifically binds to the CD25 subunit of the IL-2R, which is expressed on activated T cells. This binding prevents the IL-2 molecule from interacting with its receptor, resulting in the inhibition of T cell proliferation and activation. The IL-2/IL-2R interaction is crucial for the growth and survival of T cells, and daclizumab's mechanism of action is based on disrupting this pathway.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because daclizumab does not act through the CD28 receptor, which is involved in T cell co-stimulation.
**Option B:** This option is incorrect because daclizumab does not inhibit the enzyme cyclophosphamide, which is a chemotherapeutic agent used to treat various types of cancer.
**Option C:** This option is incorrect because daclizumab does not act through the CTLA-4 receptor, which is involved in T cell regulation and tolerance.

**Clinical Pearl / High-Yield Fact**
Daclizumab is used to treat multiple sclerosis and has been shown to reduce the frequency of relapses and slow disease progression. It is an example of a biologic agent that targets a specific component of the immune system to modulate disease activity.

**Correct Answer: C. Inhibiting T cell activation and proliferation by blocking the IL-2/IL-2R interaction.**