A network administrator upgrades the network infrastructure from copper wiring to fiber connections to increase...

The most likely issue is transposed TX/RX. Fiber connections require the transmit strand on one side to connect to the receive strand on the other side, and vice versa. If transmit is connected to transmit and receive is connected to receive, the physical termination may be correct, but the link will not come online because the devices cannot properly send and receive optical signals.

The CompTIA Network+ N10-009 objectives include fiber media, transceivers, connectors, cabling issues, and troubleshooting physical connectivity. A visual fault locator can confirm continuity and help identify breaks, bends, or termination problems, but it does not always prove that the transmit and receive strands are correctly crossed between endpoints.

An incorrect SFP could also cause a fiber link failure, but the question emphasizes that only one connection remains offline after most links came online and that the termination was verified. That makes a polarity issue, such as reversed transmit and receive strands, the strongest answer. Incompatible cable is less likely if similar connections are working. CRC errors usually indicate a link is active but experiencing frame errors, not completely offline. Therefore, transposed TX/RX is correct.