The structure labelled l in the diagram is the pituitary gland, which is a major endocrine gland in the body. The function of the pituitary gland is to regulate the activities of other endocrine glands, including the thyroid gland, the adrenal glands, and the ovaries or testes. The pituitary gland produces several hormones that control the secretion of hormones from these other glands. For example, the pituitary gland produces thyroid-stimulating hormone (TSH), which stimulates the thyroid gland to produce thyroid hormones. It also produces adrenocorticotropic hormone (ACTH), which stimulates the adrenal glands to produce cortisol, a hormone that helps the body to cope with stress. The pituitary gland also produces follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are involved in the development of secondary sexual characteristics and the regulation of the reproductive system. Therefore, the correct answer is that the function of the structure labelled l is to regulate the activities of other endocrine glands.