Final answer:
The submandibular glands, which contain both serous and mucous cells, open at the base of the lingual frenulum in the oral cavity.
Step-by-step explanation:
The type of gland that contains both serous and mucous cells is the submandibular gland, and the sublingual glands open in the oral cavity at the base of the lingual frenulum. Therefore, the correct option is b) Submandibular glands; at the base of the lingual frenulum.
The explanation for this is that the submandibular glands have a mix of serous and mucous cells, which allows them to secrete saliva that is both enzymatic and mucus-rich. This saliva enters the mouth through the submandibular ducts. In contrast, the sublingual glands mostly contain mucous cells and release their saliva via numerous small ducts called lesser sublingual ducts into the floor of the oral cavity beneath the tongue. While the parotid gland, which is responsible only for serous cell secretion of a watery saliva with salivary amylase, releases its secretion into the mouth next to the second upper molar tooth.