**Core Concept**
Ubiquitin is a small, 76-amino acid protein that plays a crucial role in various cellular processes, including protein degradation, cell cycle regulation, and DNA repair. It acts by tagging proteins with a polyubiquitin chain, marking them for proteasomal degradation. This process is essential for maintaining protein homeostasis and regulating cellular responses to stress and damage.

**Why the Correct Answer is Right**
The correct answer is related to the function of ubiquitin in marking proteins for degradation. This process is mediated by the ubiquitin-proteasome pathway (UPP), which is a critical mechanism for regulating protein turnover and quality control. The UPP involves the covalent attachment of ubiquitin to target proteins, which are then recognized and degraded by the 26S proteasome. This process is essential for maintaining protein homeostasis and regulating cellular responses to stress and damage.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the primary function of ubiquitin. While ubiquitin is involved in various cellular processes, its primary role is in marking proteins for degradation.

**Option B:** This option is incorrect because it does not accurately describe the mechanism of ubiquitin-mediated protein degradation. While ubiquitin is attached to target proteins, this process is not simply a matter of attaching a tag to a protein.

**Option C:** This option is incorrect because it does not accurately describe the role of ubiquitin in cellular processes. While ubiquitin is involved in various cellular processes, its primary role is in marking proteins for degradation.

**Clinical Pearl / High-Yield Fact**
One key point to remember is that the ubiquitin-proteasome pathway is a critical mechanism for regulating protein turnover and quality control. This pathway is essential for maintaining protein homeostasis and regulating cellular responses to stress and damage.

**Correct Answer: C. Ubiquitin is involved in marking proteins for degradation.**