Answer: d. insulin
Actually, there are two hormones secreted by islet cells within the pancreas: insulin and the glucagon. However, their secretions in response to blood sugar levels, are in opposite fashion!
Insulin is normally secreted by the beta cells of the pancreas while glucagon by the alpha cells. The amount of insulin secreted into the blood increases as the blood glucose rises. In the same way, as the blood glucose falls, the amount of insulin secreted by the pancreas goes down.
In glucagon, it is opposite. If blood glucose is high, then there is no glucagon secreted. Therefore, the answer is insulin.