* The question asks about the most likely cause of a switchport sharing issue on a Layer 2 switch.
* The answer is not A, because Spanning Tree Protocol (STP) is a protocol that prevents loops in a switched network by blocking redundant links. It does not affect the ability of users to connect to a switchport, unless the port is in a blocking state due to a loop detection.
* The answer is not B, because VLAN trunking is a method of carrying multiple VLANs over a single link, such as a switchport. It does not prevent users from connecting to a switchport, unless the port is configured with an incompatible trunking protocol or VLAN ID.
* The answer is not D, because Dynamic ARP Inspection (DAI) is a security feature that validates ARP packets and prevents ARP spoofing attacks. It does not block users from connecting to a switchport, unless the port is configured with an incorrect IP-MAC binding or DHCP snooping database.
* The answer is C, because port security is a security feature that limits the number and type of MAC addresses that can access a switchport. It can prevent users from connecting to a switchport, if the port is configured with a maximum MAC address limit that is reached by the first user, or with a static MAC address that does not match the subsequent users.
References:
* : CompTIA Network+ N10-008 Study Guide, Chapter 3: Network Components, Section 3.1: Network Devices, p. 125
* : CompTIA Network+ N10-008 Study Guide, Chapter 3: Network Components, Section 3.1: Network Devices, p. 126
* : CompTIA Network+ N10-008 Study Guide, Chapter 3: Network Components, Section 3.1: Network Devices, p. 128
* : CompTIA Network+ N10-008 Study Guide, Chapter 6: Network Security, Section 6.3: Network Security Devices and Technologies, p. 342