Option D is correct because all mentioned factors (chemical composition of saliva with high calcium and phosphate concentration, high viscosity and mucous nature of saliva, and anatomical duct characteristics that impede flow such as length and upward course against gravity) combine to make the submandibular gland more prone to stone formation, accounting for up to 80% of salivary calculi cases. | Option A is partially correct but not the only factor. | Option B is partially correct but not the only factor. | Option C is partially correct but not the only factor. | Option E is incorrect because all mentioned factors are scientifically established. | ENT surgical or sialendoscopy consultation is required when conservative treatment (gland massage, sialogogues, antibiotics) fails within two weeks, or when inflammatory episodes recur, to assess need for sialendoscopy or surgical excision. | The answer would change if the question asked for the single most important factor rather than all contributing factors. | The combination of chemical, physical, and anatomical factors explains why submandibular stones represent the vast majority of salivary calculi cases despite the parotid gland being larger. | Salivary Gland Disorders.pdf — p. 130; Sialendoscopy for Salivary gland diseases.pdf — pp. 8–10