**Core Concept**
Autophagy is a natural process where cells recycle damaged or dysfunctional components, such as proteins and organelles, to maintain cellular homeostasis. It plays a crucial role in various physiological and pathological processes, including cell development, differentiation, and response to stress.

**Why the Correct Answer is Right**
LC3 (Microtubule-associated protein 1A/1B light chain 3) is a well-established marker for autophagy. During autophagy, LC3 is conjugated to phosphatidylethanolamine (PE) to form LC3-II, which is then recruited to the autophagosomal membrane. This process is a key indicator of autophagic activity, making LC3-II a useful marker for detecting autophagy in cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Beclin-1 is a protein involved in autophagy regulation, but it is not a direct marker for autophagy. While Beclin-1 is essential for autophagosome formation, its presence does not necessarily indicate ongoing autophagy.
**Option B:** p62 is a protein that can be degraded through autophagy, but its presence does not directly indicate autophagic activity. Elevated p62 levels can be a marker of impaired autophagy, rather than autophagy itself.
**Option C:** Cathepsin D is a lysosomal enzyme involved in protein degradation, but it is not specific to autophagy. Cathepsin D is also present in non-autophagic lysosomes, making it an unsuitable marker for detecting autophagy.

**Clinical Pearl / High-Yield Fact**
Autophagy plays a crucial role in maintaining cellular homeostasis and responding to stress. Dysregulation of autophagy has been implicated in various diseases, including cancer, neurodegenerative disorders, and metabolic disorders.

**Correct Answer: C. LC3**