This describes a Spanning Tree Protocol (STP) loop. If STP isn't correctly configured or a redundant link is added without STP protection, it causes broadcast storms and network outages.
A). Unused ports disabled would not affect the entire network.
B). Missing default gateway on a switch doesn't cause total network loss.
**C. Connecting a switch to an access port can cause VLAN mismatches, but not total connectivity loss unless a loop forms.
# Reference:
CompTIA Network+ N10-009 Official Objectives: 3.6 - Explain the characteristics of network topologies and types.