**Core Concept**
The question tests the understanding of **immunology**, specifically the role of **antigen-presenting cells (APCs)** in the immune system. Professional APCs are crucial for initiating immune responses by processing and presenting **antigens** to **T-cells**.

**Why the Correct Answer is Right**
Professional APCs, such as **dendritic cells** and **macrophages**, are specialized to capture, process, and present antigens to T-cells, thereby activating the adaptive immune response. These cells express **major histocompatibility complex (MHC)** molecules, which are essential for antigen presentation.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while **B-cells** can present antigens, they are not the primary professional APCs.
**Option B:** Incorrect as **keratinocytes** are not primarily antigen-presenting cells.
**Option D:** Incorrect because **platelets** do not function as professional APCs.

**Clinical Pearl / High-Yield Fact**
Professional APCs play a critical role in initiating immune responses, and their dysfunction can lead to **immunodeficiency** or **autoimmune diseases**.

**Correct Answer:** Dendritic cells and macrophages are the primary professional antigen-presenting cells, but without the exact answer choice provided, the most accurate choice based on common options would be: **Correct Answer: D. Dendritic cells.**