The best solution is to implement authentication for API requests and apply appropriate rate limiting (C). Authentication ensures that only authorized customers or systems can access the API, while rate limiting helps prevent denial-of-service (DoS)-like conditions and cost inflation from excessive or malicious requests. This addresses both the security (unauthorized access) and cost issues (serverless billing based on execution).

Option A (digital certificates) is a strong control for authentication but may introduce unnecessary complexity and does not address rate abuse directly. Option B (rate limits with IP reputation checks) is useful but insufficient—malicious actors may rotate through new IPs not yet flagged. Option D (regional restrictions) might reduce some noise traffic but risks blocking legitimate global users and is not scalable for a modern cloud service.

CAS-005 highlights securing APIs with authentication, authorization, and throttling as best practices. Thus, the combined approach of API authentication plus rate limiting is the most comprehensive and effective solution.