D). Crimper: A crimper is used to terminate an RJ45 cable properly by attaching the connector to the twisted-pair wires. If the termination is not done correctly, the connection will be intermittent or fail entirely. Incorrect Options: A). Toner probe: Used to locate cables or trace their path, not for terminating RJ45 connectors. B). Cable tester: Useful for testing connectivity but does not fix termination issues. C). Punchdown: Used to connect wires to a patch panel or keystone jack, not for RJ45 connectors. Key Takeaway: A crimper is the proper tool for fixing an improperly terminated RJ45 connection. Reference: CompTIA A+ Core 1 Exam Objectives (220-1101), Domain 2.8 - Using appropriate tools for network troubleshooting.