Option C cannot be the cause because the password used to log in to the ECS operating system has no requirement to match the database credential used by an RDS instance. ECS authentication and RDS database authentication are separate security domains. An application running on an ECS connects to RDS using the database endpoint, port, database username, and corresponding database password.

Option A can cause the failure. Huawei's RDS troubleshooting guidance requires appropriate security-group rules so that traffic to the database port is permitted.

Option B can also prevent communication. Huawei Cloud states that when an ECS and RDS DB instance are deployed in different VPCs, network connectivity must be established—for example through an appropriate VPC peering connection —or the resources must be placed in a mutually reachable network.

Option D can likewise cause connection failure. Huawei's official RDS troubleshooting process specifically instructs administrators to validate the connection address and port configuration and test connectivity to the RDS listening port. Examples include port 3306 for RDS for MySQL and 5432 for PostgreSQL.

The ECS's own login password is unrelated to these connectivity requirements; therefore C is correct.

Reference topics: RDS Database Connectivity; ECS-to-RDS Communication; Security Groups; VPC Peering; Database Ports.

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