**Core Concept**
Reactive T immunoblasts are activated T cells that proliferate in response to an antigenic stimulus, such as a viral infection. These cells play a key role in the immune response, particularly in the lymphoid organs.

**Why the Correct Answer is Right**
Reactive T immunoblasts are typically found in the paracortical region of the lymph node, which is rich in T cells. This region is characterized by a dense network of T cell zones, where activated T cells interact with antigen-presenting cells (APCs) and other T cells. The paracortical region is the site of intense immune activity, where T cells proliferate and differentiate in response to antigenic stimuli. The presence of prominent nucleoli in the reactive T cells suggests active cell division, which is consistent with the paracortical region's role in immune activation.

**Why Each Wrong Option is Incorrect**
**Option A:** The medullary cords are primarily composed of B cells and plasma cells, which are involved in antibody production. While T cells may be present in the medullary cords, they are not the primary cell type.
**Option B:** The subcapsular sinus is a region of the lymph node where antigens are filtered and presented to APCs. However, it is not a site of T cell proliferation.
**Option C:** The lymphoid follicles are primarily composed of B cells and are involved in the germinal center reaction. While T cells may be present in the follicles, they are not the primary cell type.

**Clinical Pearl / High-Yield Fact**
Activated T cells, such as reactive T immunoblasts, can be found in the paracortical region of lymphoid organs, including lymph nodes, spleen, and tonsils. This region is a key site of immune activation and cell-mediated immunity.

**Correct Answer: C.**