Step 1: Recall the syndrome features. Gardner's syndrome is a variant of familial adenomatous polyposis (FAP) characterized by colonic adenomatous polyps plus extra-intestinal features: multiple epidermoid/sebaceous cysts, osteomas, and desmoid tumours.
Step 2: Match the clue. The presence of multiple sebaceous (epidermoid) cysts is the classic skin finding of Gardner's syndrome, making option A correct.
Step 3: Eliminate the other options. Turcot syndrome combines colonic polyposis with central nervous system tumours (medulloblastoma/glioma). Muir-Torre syndrome features sebaceous adenomas/carcinomas with visceral malignancies, not multiple simple sebaceous cysts. Cowden syndrome is associated with multiple hamartomas, trichilemmomas, and breast/thyroid cancers.
Step 4: Conclusion. Multiple sebaceous cysts are seen in Gardner's syndrome.