Insulin is a hormone which

 Insulin reduces the body's blood sugar levels.

Insulin lowers blood sugar and delivers glucose for energy by assisting cells in absorbing glucose. The pancreas secretes glucagon when blood sugar levels are too low. Glucagon signals the liver to release stored glucose, causing blood sugar levels to increase. As a result, insulin is a hormone that lowers blood sugar levels.

Hence, the correct answer is option D) Decrease blood sugar.