The correct answer is A. Nessus.

Nessus is a widely used tool for system vulnerability testing. It helps identify and assess vulnerabilities in various systems and networks. It performs scans to detect potential weaknesses and provides detailed reports on the vulnerabilities found. Nessus is known for its extensive vulnerability database and its ability to perform comprehensive security assessments.

Options B, C, and D are not specifically designed for system vulnerability testing:

B. HP Web Inspect: HP Web Inspect is a web application security testing tool that focuses on identifying vulnerabilities in web applications, rather than system vulnerabilities.

C. TAM: It is unclear what "TAM" refers to in this context. Without more information, it is difficult to determine if it is a tool suitable for system vulnerability testing.

D. SDL: SDL stands for "Security Development Lifecycle," which is a methodology for developing secure software. It is not a specific tool used for system vulnerability testing.

Therefore, option A, Nessus, is the most appropriate tool for system vulnerability testing.