The correct answer is nmap.

nmap is purpose built for network discovery and mapping. It performs host discovery and port and service scans, it can fingerprint operating systems, and it exports results in formats that feed visualization tools so an analyst can build an accurate inventory of active hosts and a topology map.

OpenVAS is primarily a vulnerability scanner and it focuses on finding and reporting vulnerabilities rather than on broad host discovery and topology mapping. It may find hosts as part of vulnerability scans but it is not optimized for inventorying and mapping a network.

Nessus is also a vulnerability assessment tool and it is designed for identifying vulnerabilities and compliance issues. It is not the ideal choice when the main goal is to enumerate active hosts and produce a topology diagram.

sqlmap is an automated SQL injection testing tool for web applications and it does not provide general network host discovery or topology mapping capabilities. It is unrelated to the discovery task described in the question.

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