## Core Concept
The concept of a "second signal" in B-cell activation refers to the requirement for B cells to receive two signals to become fully activated. The first signal is provided by the binding of the B cell receptor (BCR) to the specific antigen. The second signal, also known as co-stimulation, is necessary to prevent anergy or tolerance and to proceed with activation, proliferation, and differentiation into antibody-secreting plasma cells.

## Why the Correct Answer is Right
The correct answer involves understanding the mechanisms through which B cells receive their second signal. This can occur through several pathways, but a key one involves the interaction between CD40 on the B cell and CD40 ligand (CD40L) on T cells. This interaction is crucial for class switching, germinal center formation, and long-lived immunity.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, any option not directly related to known second signal mechanisms (like CD40-CD40L interaction) would be incorrect.
- **Option B:** Similarly, without specifics, any option not aligning with established co-stimulatory pathways for B cells would be incorrect.
- **Option C:** This would be incorrect if it does not accurately represent a known mechanism for providing a second signal to B cells.

## Clinical Pearl / High-Yield Fact
A critical clinical pearl is that the CD40-CD40L interaction is not only crucial for B-cell activation but also a target for therapy in certain autoimmune diseases and for preventing transplant rejection. The lack of this interaction (e.g., in CD40L deficiency) leads to immunodeficiency characterized by increased susceptibility to infections and defective antibody responses.

## Correct Answer Line
**Correct Answer: D.**