D  
Larger filler particles present less total surface area per unit volume, so the resin needs to wet a smaller surface; this lowers inter-particle friction and makes the uncured paste flow more easily, i.e., viscosity falls [Lee, 2006, PMID 16171856]. Studies also show that as particle size increases, strength actually drops rather than rises, while surface roughness increases, ruling out the other options [Tanimoto, 2006, PMID 16950668].