Final answer:
Fibroblasts are the cells that secrete proteins to form the matrix of loose connective tissue, with collagen being the most prevalent protein in the matrix.
Step-by-step explanation:
The type of cell that secretes the proteins that make up the matrix of loose connective tissue is the fibroblast. These cells are responsible for producing the extracellular matrix, which includes a range of proteins such as collagen, elastin, and reticular fibers. Collagen is the most abundant protein in mammals, comprising about 25% of the total protein content in these organisms. The matrix produced by fibroblasts not only provides structural support to tissues but also plays a role in cellular communication within the tissue.