John works as a professional Ethical Hacker. He has been assigned the project of testing the attack phase successfully: Information gathering Determination of network range Identification of active…

OS fingerprinting is the next logical step after identifying active systems and open ports because it determines the specific operating systems running on target machines, which is crucial for identifying OS-specific vulnerabilities and planning targeted exploits.

Mapping the network involves determining the network range and identifying active systems, which the question states has already been performed as part of the initial information gathering.

Installing a backdoor is an exploitation or post-exploitation activity, which occurs much later in the ethical hacking process after vulnerabilities have been identified and exploited.

Fingerprinting services running on open ports is a part of identifying open ports and applications, which the question states has already been completed.