**Core Concept**
Microfilaments are a type of cytoskeletal structure found in eukaryotic cells, playing a crucial role in cell shape, division, and intracellular signaling. They are dynamic structures that can rapidly assemble and disassemble in response to various cellular signals.

**Why the Correct Answer is Right**
Microfilaments are composed of two identical subunits of the protein actin. Actin polymerization is a highly regulated process, involving the binding of actin monomers to the plus end of the microfilament, followed by the elongation of the filament through the addition of more actin subunits. This process is essential for various cellular functions, including muscle contraction, cell migration, and cytokinesis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because microtubules, not actin filaments, are composed of tubulin subunits and are involved in maintaining cell shape, organizing microtubule-associated proteins, and forming the mitotic spindle.
**Option B:** This option is incorrect because intermediate filaments are composed of a variety of proteins, including keratins, lamin, and vimentin, and are involved in maintaining cell shape and providing mechanical support to cells.
**Option C:** This option is incorrect because microfilaments are not composed of myosin, which is a motor protein involved in muscle contraction and intracellular transport.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the actin cytoskeleton is highly dynamic and can be rapidly reorganized in response to various cellular signals, making it a key player in many cellular processes, including muscle contraction, cell migration, and cytokinesis.

**Correct Answer: D. Actin.**