**Core Concept**
The question is testing the student's knowledge of the most abundant protein in mammalian cells, which plays a crucial role in maintaining cellular structure and function.

**Why the Correct Answer is Right**
Actin is the most abundant protein in mammalian cells, making up approximately 20-30% of total cellular protein. It is a crucial component of the cytoskeleton, providing structural support, shape, and movement to cells. Actin filaments (F-actin) are dynamic structures that can be rapidly assembled and disassembled, allowing for changes in cell shape and movement.

**Why Each Wrong Option is Incorrect**
**Option A:** Myosin is a motor protein that interacts with actin filaments to produce muscle contraction, but it is not the most abundant protein in mammalian cells.

**Option B:** Tubulin is a crucial component of microtubules, which are essential for maintaining cellular structure, organizing microtubule-associated proteins, and serving as tracks for motor proteins. However, tubulin is not the most abundant protein in mammalian cells.

**Option C:** Keratin is a type of intermediate filament protein that provides structural support to epithelial cells, but it is not the most abundant protein in mammalian cells.

**Clinical Pearl / High-Yield Fact**
The actin cytoskeleton plays a crucial role in maintaining cellular shape, movement, and division. Disruptions in actin dynamics have been implicated in various diseases, including cancer, muscular dystrophy, and cardiovascular disease.

**Correct Answer:** C. Actin.