What intelligence collection method do investigators use to monitor a suspect's social network feeds?

The correct choice revolves around the concept of open-source intelligence (OSINT), which refers to gathering and analyzing information from publicly available sources. Social network feeds, such as posts, comments, photos, and user interactions, are readily accessible online and fall under the category of open-source information.

Investigators utilize this method to monitor a suspect's activities, behaviors, and connections in real time. This type of intelligence collection is valuable because it can provide insights into a subject's personal life, affiliations, and potential criminal behavior without requiring clandestine operations or breaching privacy laws.

The other methods mentioned, while related to information gathering, do not specifically pertain to the direct monitoring of social network feeds. For example, human intelligence focuses more on personal interactions, interviews, and direct information obtained from informants, rather than publicly available content online. The deep web refers to parts of the internet not indexed by standard search engines but does not inherently focus on social media or open-platform networks. Intelligence feeds involve the collection of data for analysis but do not specifically capture the essence of monitoring social networking sites. Thus, open-source intelligence is indeed the most appropriate and targeted method mentioned in the question.