**Core Concept**
The question is testing knowledge of checkpoint inhibitors, specifically monoclonal antibodies targeting the PD-L1 protein. PD-L1 (Programmed Death-Ligand 1) is a protein that, when expressed on tumor cells, can suppress the immune system by interacting with its receptor PD-1 on T cells.

**Why the Correct Answer is Right**
Atezolizumab is a monoclonal antibody that targets PD-L1, preventing its interaction with PD-1 on T cells. This blockade enhances T cell activity, leading to increased anti-tumor immune response. Atezolizumab is used in the treatment of non-small cell lung cancer, bladder cancer, and other malignancies. By inhibiting the PD-L1/PD-1 axis, atezolizumab helps restore the immune system's ability to recognize and attack cancer cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Nivolumab is a monoclonal antibody against PD-1, not PD-L1. While it also targets the PD-1/PD-L1 axis, it binds to PD-1 on T cells rather than PD-L1 on tumor cells.
**Option B:** Pembrolizumab is another monoclonal antibody targeting PD-1, similar to nivolumab. It is used in the treatment of various cancers, including melanoma and non-small cell lung cancer, but it does not target PD-L1.
**Option C:** Ipilimumab is a monoclonal antibody against CTLA-4, a different checkpoint receptor on T cells. While it also enhances anti-tumor immune response, it targets a different protein and is not a PD-L1 inhibitor.

**Clinical Pearl / High-Yield Fact**
Atezolizumab has shown efficacy in combination with chemotherapy in the treatment of non-small cell lung cancer, making it an important option for patients with this diagnosis. It is essential to remember that PD-L1 expression levels can impact treatment decisions and outcomes with PD-L1 inhibitors like atezolizumab.

**Correct Answer:** C. Atezolizumab.