**Core Concept**
Memory cells, also known as memory T cells or memory B cells, are a type of immune cell that retains a 'memory' of previously encountered pathogens, allowing for a rapid and effective immune response upon subsequent exposure.

**Why the Correct Answer is Right**
Memory cells play a crucial role in long-term immunity by recognizing and responding to specific antigens that have been encountered before. This is achieved through the activation of memory T cells, which express specific T-cell receptors (TCRs) that recognize and bind to antigens presented by antigen-presenting cells (APCs). The activation of memory T cells leads to the rapid proliferation and differentiation of effector T cells, which mount an effective immune response against the pathogen.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specifically describe the function of memory cells.
**Option B:** This option is incorrect as it refers to effector T cells, which are responsible for the initial immune response, rather than memory cells.
**Option C:** This option is incorrect as it refers to naive T cells, which have not encountered a pathogen before and require activation to become effector T cells.

**Clinical Pearl / High-Yield Fact**
Memory cells can provide long-term immunity against pathogens, such as tetanus and measles, which is why booster shots are often required to maintain immunity.

**Correct Answer:** C. Naive T cells