Final answer:
Animal cells with extensive extracellular matrix primarily synthesize collagen, the main structural protein providing strength and support.
Step-by-step explanation:
Animal cells that produce an extensive extracellular matrix are primarily involved in the synthesis of structural proteins that hold tissues together and provide mechanical support. Among the options provided, the ribosomes in these cells would be expected to synthesize large amounts of collagen, which is the most abundant protein in the extracellular matrix and provides strength and structure to various tissues. In contrast, hemoglobin is involved in oxygen transport in the blood, actin is a part of the cytoskeleton and involved in cell movement and shape, and insulin is a hormone involved in glucose metabolism but these are not structural components of the extracellular matrix.