**Core Concept**
BIS-GMA (Bisphenol A-glycidyl methacrylate) is a resin monomer commonly used in dental composites. It is a dimethacrylate, which undergoes a polymerization reaction to form a cross-linked network. This cross-linking provides the necessary strength and durability to dental restorations.

**Why the Correct Answer is Right**
BIS-GMA undergoes a free radical polymerization reaction, initiated by camphorquinone, a photoinitiator. This reaction involves the formation of a polymer network through the reaction of methacrylate groups. The cross-linking of BIS-GMA molecules provides the necessary mechanical properties to dental composites, such as compressive strength and durability.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the properties of BIS-GMA. While BIS-GMA does undergo a polymerization reaction, it is not primarily used as a filler material in dental composites.
**Option B:** This option is incorrect because it is a common filler material used in dental composites, such as silica or glass particles, but it does not describe the properties of BIS-GMA.
**Option C:** This option is incorrect because it describes the properties of another type of resin monomer, urethane dimethacrylate (UDMA), which is also used in dental composites but has different properties than BIS-GMA.

**Clinical Pearl / High-Yield Fact**
BIS-GMA is a common allergen and has been linked to allergic contact dermatitis in dental professionals. This is an important consideration when handling dental composites containing BIS-GMA.

**Correct Answer: D. BIS-GMA is a dimethacrylate resin monomer commonly used in dental composites for its strength and durability.**