The core requirement is to identify who logged into a system during a specific period for a security investigation. This task is the primary function of the AAA (Authentication, Authorization, and Accounting) framework. The Accounting component of AAA is specifically designed to collect and log data about user activity, including session start/stop times, commands executed, and resources accessed. These logs are essential for security audits, forensics, and troubleshooting, directly addressing the engineer's need to identify user logins during the breach.

A. SNMP: Simple Network Management Protocol (SNMP) is used for monitoring and managing network devices' operational status and performance, not for auditing user access to application systems.

B. NACM: NETCONF Access Control Model (NACM) is a specific standard used to control user access to network configuration and state data via the NETCONF protocol, not a general system access framework.

D. PKI: Public Key Infrastructure (PKI) is a framework for managing digital certificates and public-key encryption. While it can be used for authentication, it does not provide the accounting/logging function needed here.

1. Cisco IOS XE Security Configuration Guide, Cisco IOS XE Bengaluru 17.6.x - AAA Overview: "Accounting provides a method for collecting and sending security server information used for auditing, billing, and reporting... When AAA accounting is enabled, the network access server reports user activity to the TACACS+ or RADIUS security server". This directly supports the use of AAA for auditing user activity.

2. Cisco Identity Services Engine Administrator Guide, Release 3.1 - Chapter: Reports > View and Manage Reports: "Cisco ISE provides a robust logging mechanism that you can use to create reports of all activity on your network... You can generate reports for authentication, authorization, and accounting (AAA) events." This shows how a Cisco AAA implementation (ISE) is used for reporting on access events.

3. RFC 2866 - RADIUS Accounting: Section 1, "Introduction," states, "This document describes the RADIUS Accounting protocol. This protocol can be used to convey information about how a user is making use of a resource." This foundational document establishes accounting as the mechanism for tracking user resource usage, including logins.

4. Cisco IOS XE SNMP Configuration Guide, Cisco IOS XE Gibraltar 16.12.x - SNMP Overview: "The Simple Network Management Protocol (SNMP) is an application-layer protocol that provides a message format for communication between SNMP managers and agents. SNMP provides a standardized framework and a common language used for the monitoring and management of devices in a network." This reference confirms SNMP's role is network management, not user access auditing.