Which major salivary glands are located outside the oral cavity just inferior to the floor of mouth and have relatively long excretory ducts that open on either side of the lingual frenulum?

Submandibular glands

Explanation:

The submandibular glands are a pair of major salivary glands which are outside teh oral cavity just inferior to the floor mourth or beneath the lower jaws.

Submandibular glands secrets a mixture of mucus and serous fluid which enters the oral cavity through the submandibular duct, which opens on either side of the lingual frenulum.

Hence, the correct answer is Submandibular glands.