The most likely cause is that the switch ports were previously configured for a different VLAN than 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.