C

Both alginate (irreversible hydrocolloid) and agar-agar (reversible hydrocolloid) form elastic gels composed of a network of fibrils; when the set impression is pulled over undercuts these gel fibers must stretch and then recover, so any distortion that occurs during removal is due to deformation of the gel-fiber network in both materials [Davidson & Bekke-Hoekstra, 1979, PMID 293792; Johnson & Craig, 1986, PMID 3456041]. The two materials differ in their temperature behavior (agar gels on cooling, alginate sets faster at higher temperature), setting manipulation, and re-usability, making the other options incorrect.