1. **Problem statement:** Given the signatures ACA=256, ACB=777, and ACC=1984, if C wants to prove its identity to A, what message should C send? 2. **Understanding the problem:** The signatures represent some form of cryptographic or identity verification values. To prove identity, C must send a message that matches the signature expected by A. 3. **Analyzing the signatures:** The signature ACA=256 likely represents the signature of A on C's identity or message. 4. **Conclusion:** Since C wants to prove its identity to A, C should send the signature ACA=256 as the message to A. **Final answer:** 256