Insulin is a hormone that is produced by the pancreas in the body. It plays a crucial role in regulating blood sugar levels. When we consume carbohydrates, the body breaks them down into glucose, which enters the bloodstream. Insulin is then released to help the cells take in the glucose and use it as energy.

Option 1: Hormone. This is the correct answer. Insulin is classified as a hormone because it is produced in one part of the body (pancreas) and acts on another part of the body (cells).

Option 2: Protein. While insulin is a type of protein, it is more specifically classified as a hormone.

Option 3: Enzyme. Insulin is not an enzyme. Enzymes are proteins that facilitate chemical reactions in the body.

Option 4: Vitamin. Insulin is not a vitamin. Vitamins are organic compounds required in small quantities for various metabolic processes.

In conclusion, insulin is a hormone that helps regulate blood sugar levels in the body.