A cable crimper is the correct answer because it is the essential tool used for attaching RJ45 connectors to the ends of a CAT6 cable to create a custom Ethernet cable. The crimper's purpose is to securely fasten the connector to the cable, ensuring that each of the individual wires within the cable makes good contact with the corresponding pins inside the RJ45 connector. Without a proper crimping tool, the connectors may not be attached firmly, leading to poor connection quality or complete connectivity failure. Other tools mentioned are not primarily used for the purpose of attaching connectors to cables.