**Core Concept**
CD95, also known as Fas, is a death receptor that plays a crucial role in regulating apoptosis (programmed cell death). It is a member of the tumor necrosis factor receptor (TNFR) family and is involved in the induction of apoptosis in various cell types.

**Why the Correct Answer is Right**
When a cell is activated by its ligand, FasL, the interaction between CD95 and its ligand triggers a cascade of intracellular signaling events that ultimately lead to the activation of caspases and the execution of apoptosis. This process is essential for maintaining immune homeostasis and preventing excessive cell growth. The Fas/FasL pathway is also involved in the regulation of immune responses and the elimination of self-reactive lymphocytes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because CD95 is not a marker of cell proliferation. While it is involved in the regulation of cell growth, its primary function is the induction of apoptosis.
**Option B:** This option is incorrect because CD28 is a costimulatory molecule involved in T cell activation, not a marker of apoptosis.
**Option C:** This option is incorrect because B7-1 is a ligand for CD28, not a marker of apoptosis.

**Clinical Pearl / High-Yield Fact**
The Fas/FasL pathway is a critical mechanism for maintaining immune tolerance and preventing autoimmune diseases. Dysregulation of this pathway has been implicated in various conditions, including autoimmune disorders and cancer.

**Correct Answer:** D. Apoptosis marker