Which tool is unable to identify a target's operating system for penetration testing?

In the context of penetration testing, identifying a target's operating system is critical for understanding potential vulnerabilities and determining the best approach for exploitation. The correct answer is sqlmap, as it is specifically designed for automating the process of detecting and exploiting SQL injection vulnerabilities in web applications. While sqlmap does a great job at identifying and exploiting database vulnerabilities, it is not equipped with features to discern the operating system of a target.

On the other hand, Nmap is a powerful network scanner that can actively probe a target system to determine its operating system based on various techniques, including TCP/IP stack fingerprinting. Netcat, often referred to as the "Swiss Army knife" of networking, is capable of establishing connections and can sometimes infer the operating system through response behavior, although this is not its primary function. Wireshark, as a network protocol analyzer, lets users capture and examine data packets flowing through a network, which can also offer insights into the target's operating system through the analysis of network traffic.

Thus, sqlmap's specialized focus on SQL injection makes it unsuitable for the broader task of operating system identification, which distinguishes it as the correct choice in this scenario.