What is an active method for information gathering in network security?

Identifying valid email addresses is considered an active method for information gathering in network security because it involves direct interaction or inquiry that can lead to obtaining specific and actionable information about an organization's personnel or systems. This method often includes techniques such as guessing email patterns or utilizing tools to verify the existence of email addresses associated with a domain. By doing so, security professionals can glean insights into the organization's structure, potentially uncover vulnerabilities related to social engineering attacks or phishing attempts.

In contrast, the other choices are primarily passive techniques. Locating an organization's web presence and reviewing public social media profiles involve gathering publicly available information without directly engaging with the target system or individuals. Additionally, examining domain registration data can also be considered passive, as it typically consists of looking up WHOIS information that has already been made publicly accessible, rather than actively probing the organization itself.