Correct Answer: DTo check the usage of the source system hardware such as CPU, memory, and network in an SAP system, the appropriate transaction to run from the SAP GUI is OS07N. This transaction is specifically designed for monitoring the operating system's activity and provides detailed statistics on the hardware usage. Other options like SM51, DB01, and DB12 serve different purposes, such as displaying application servers, analyzing exclusive lock waits, and monitoring database backups respectively, and are not suitable for this requirement.