Which of the following is an example of passive reconnaissance?

Passive reconnaissance involves gathering information without actively engaging with the target system or network, thus minimizing the risk of detection. Observing network traffic falls under this category since it allows one to collect data about the network's activities and communications without initiating any direct interaction with the network devices or systems. This method can provide valuable insights into the types of protocols in use, the volume of traffic, and potential vulnerabilities, all without raising alarms.

In contrast, other examples like port scanning and network mapping are considered active reconnaissance techniques, as they involve direct probing of the network to gather information, which can alert the target to the reconnaissance effort. Social engineering, while it can sometimes involve less direct techniques, generally also pertains to active engagement, as it often requires interaction with personnel or systems to elicit information.