Concept:An increase in the diameter of a plant stem is caused by the cell division of lateral meristematic tissue, which is the cambium.Explanation:The growth in stem width is called secondary growth. Secondary growth occurs when lateral meristem tissues divide actively. In dicot stems, the cambium divides to form new xylem and phloem, which adds thickness to the stem. Phloem and xylem are transport tissues and do not divide to increase diameter. Collenchyma is a supporting tissue and does not cause secondary growth.Answer:A. cambium