Final answer:
The primary activity of glandular epithelium is secretion, as these cells specialize in producing and releasing substances such as enzymes, hormones, and mucous.
Step-by-step explanation:
The primary activity of glandular epithelium is secretion. Glandular epithelial cells are specialized in the production and release of various substances like enzymes, hormones, and mucous. These cells form glands, which can be part of larger organs or localized collections of cells. Exocrine glands release their secretions through ducts to the exterior of the body or onto an internal space or lumen, while endocrine glands secrete hormones directly into the bloodstream. Thus, among the options given, secretion (B) is the main function of glandular epithelium.