What type of hormone is insulin, and what is its primary role in insulin signaling?

• A. Insulin is a peptide hormone that acts as a ligand to activate receptor tyrosine kinases.
• B. Insulin is a steroid hormone that acts as a ligand to activate receptor tyrosine kinases.
• C. Insulin is a steroid hormone that acts as a ligand to inhibit receptor tyrosine kinases.
• D. Insulin is a peptide hormone that acts as a ligand to inhibit receptor tyrosine kinases.

Answer: (A)