The correct display filter for analyzing FTP traffic in a PCAP file is "tcp.port==21". This filter will show all TCP packets where the port number is 21, which is the standard port for FTP control messages.

Protected data refers to any information that can be used on its own or with other information to identify, contact, or locate a single person, or to identify an individual in context. In the scenario described, the engineer must identify data that is considered protected under privacy laws and regulations. Personal Identifiable Information (PII) and Protected Health Information (PHI) are two types of data that are considered protected. PII includes any data that could potentially identify a specific individual, such as addresses and gender. PHI refers to any information about health status, provision of health care, or payment for health care that can be linked to an individual. This is what makes both PII and PHI crucial to be identified and protected in compliance with data protection regulations.

Corroborative evidence is the type of evidence that supports a theory or an assumption that results from initial evidence. It provides additional support to the initial findings, strengthening the theory or assumption by confirming the same facts or pointing towards the same conclusion with independent pieces of evidence4567.