The correct answer is a cable tester. A cable tester is used to check for physical connectivity problems in the cable such as opens (incomplete circuits) or shorts (unwanted connections). If a cable has an open or a short, the electrical signals cannot properly transit, which leads to connection issues. LED status indicators and tone generators are useful for different purposes; LED indicators help to visually confirm the operational status of the network hardware, while a tone generator is typically used to trace and locate a particular cable in a bundle, not to test for physical cable defects like opens or shorts. Incorrect transceivers would result in connectivity issues, but wouldn't be used to test for opens or shorts.