The correct option is A Somatostatin
The endocrine part of the pancreas consists of specialised tissue called the islets of Langerhans. The islets of Langerhans consist of alpha, beta and delta cells.
Insulin and glucagon are secreted by the beta and alpha cells, respectively. They are responsible for maintaining the blood glucose levels.
Somatostatin is produced by the delta cells as well as the hypothalamus. The target organs of somatostatin are the anterior pituitary and the pancreas. Somatostatin inhibits the secretion of growth hormone by the pituitary, and the secretion of insulin and glucagon by the pancreas.