**Core Concept:**
The concept being tested here is the classification of immune responses into primary and secondary responses, and their respective mediators. Primary immune response is a first-time exposure to a pathogen, which leads to the activation of naïve T cells and B cells. Secondary immune response occurs upon re-exposure to the same pathogen, involving memory T cells and memory B cells that have been formed during the primary immune response.

**Why the Correct Answer is Right:**
In the context of the question, secondary immune response refers to the response triggered when the body encounters the same antigen again after initial exposure. In this case, memory B cells and memory T cells, which have been formed during the primary immune response, are activated. These cells recognize the antigen and initiate an immune response more rapidly and efficiently than the primary response. This is because memory cells are faster to activate, proliferate, and differentiate into effector cells, leading to a quicker and more robust immune response.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because memory cells are not involved in the primary immune response.
B. This option is incorrect because memory cells are not involved in the primary immune response.
C. This option is incorrect because memory cells are not involved in the primary immune response.
D. This option is incorrect because memory cells are not involved in the primary immune response.

**Clinical Pearl / High-Yield Fact:**
A high-yield fact from this question is the distinction between primary and secondary immune responses, highlighting the role of memory cells in a more efficient immune response during secondary exposure to a pathogen. This knowledge is crucial for understanding immunology and its implications in various clinical scenarios, such as immunization, vaccine efficacy, and immune responses to reinfections or re-exposure to previously encountered pathogens.

**Correct Answer:** .