Final answer:
Insulin is secreted by beta cells located within the islets of Langerhans in the pancreas. These cells respond to high blood glucose levels by releasing insulin, which helps regulate blood glucose by promoting its absorption in tissues. Hence, the correct option among the given ones is Option D.
Step-by-step explanation:
The cells that secrete insulin are found within the pancreas, specifically in the clusters of endocrine cells known as the pancreatic islets or the islets of Langerhans. Among the different types of cells present within these islets, it is the beta cells that are responsible for the production and secretion of insulin. This hormone plays a critical role in regulating blood glucose levels by promoting the absorption of glucose into various tissues, including fat, liver, and skeletal muscle cells, where it can be stored as glycogen or converted to fats.
Insulin secretion increases when blood glucose levels rise, such as after a meal, while another hormone, glucagon, produced by alpha cells, works to raise blood glucose levels when they decline. Together, insulin and glucagon maintain blood glucose at an optimal level. Thus, the correct option for the given question is Option D.