Which of the following salivary gland tumors has highest rate of malignant transformation:

Tumours of salivary gland tissue constitute about 5% of head and neck tumors and affect major salivary glands five times more than the minor salivary glands. The incidence of malignancy among salivary gland tumours varies inversely with the size of the gland. About 15% of parotid tumors, 50% of submandibular gland tumours, and 90% of minor salivary gland tumours are malignant. Approximately 70% of all minor salivary gland cancers of oral cavity occur on the hard palate.