The sublingual salivary gland primarily produces mucous fluid. Unlike the parotid glands, which secrete mainly serous fluid, and the submandibular glands, which produce a mixed seromucous secretion, the sublingual glands are characterized by their predominantly mucous secretion.

- The sublingual gland is the smallest of the three major salivary glands and is located beneath the tongue.
- It contains mainly mucous acini, which produce thick, viscous mucous fluid that helps in lubricating the oral cavity and aiding in swallowing.
- This mucous secretion is important for maintaining moisture on the mucosal surfaces and providing a protective barrier.
- In contrast, serous fluid is watery and rich in enzymes like amylase, produced mostly by the parotid gland.
- The submandibular gland produces a mixed seromucous secretion, meaning it has both serous and mucous components.

Therefore, the correct answer is Option 2: Mucous fluid.

Reference: Gray’s Anatomy, 41st Edition, Head and Neck Section, Chapter 41, Page 688