The most likely cause is that the switchports were previously configured for a different VLANthan the one the users' computers are on. If the native VLAN on the port doesn't match the end device's VLAN, communication fails.
* A. Ports are error-disabled: Would result in no link at all, not common across multiple ports unless a violation occurred.
* C. MDIX issue: Auto-MDIX eliminates most crossover problems on modern switches.
* D. Ports are trunk ports: While possible, typical user devices should be on access ports, but if the port is incorrectly trunked, it can cause similar issues. However, "incorrect VLAN" is more precise here.
#Reference:
CompTIA Network+ N10-009 Official Objectives: 2.3 - Given a scenario, configure and verify VLANs.