An open circuit in networking typically refers to a break or disconnection in a network cable's conductive path. This causes the signal to be interrupted, leading to intermittent or complete loss of network connectivity. Since the cable tester indicated an open circuit, the most likely cause is a break in one or more of the wire connections within the cable, which could be due to physical damage or improper termination. The other options presented are related to different types of cable issues or symptoms that would not specifically cause an open circuit indicated by the cable tester.