The most probable cause of database timeout messages after an application update, with a stable number of concurrent active users, is likely related to changes in how the application manages database connections or queries. Option C is the most relevant because an increased number of connections to the Cloud SQL instance, possibly due to inefficient connection handling or query execution in the updated application, could lead to timeouts. Options A and D are less likely because serial port logs (A) are more for system-level issues and Cloud Security Scanner (D) is for identifying security vulnerabilities, not performance issues. Option B, using Stackdriver Profiler, could help identify resource utilization issues but is more general and not as directly related to database timeouts as option C.