A subnet mask is a binary number that indicates which bits of an IP address belong to the network portion and which bits belong to the host portion. A slash notation (/n) indicates how many bits are used for the network portion. A /20 notation means that 20 bits are used for the network portion and 12 bits are used for the host portion. To convert /20 to a dotted decimal notation, we need to write 20 ones followed by 12 zeros in binary and then divide them into four octets separated by dots. This gives us 11111111.11111111.11110000.00000000 or 255.255.240.0 in decimal. Reference: https://partners.comptia.org/docs/default-source/resources/comptia-network-n10-008-exam-objectives-(2-0), https://www.techopedia.com/definition/950/subnet-mask

The 10GBASE-T connection between the two buildings uses Cat 5e, which is not rated for a distance of 410ft (125m). According to the CompTIA Network+ Study Manual, for 10GBASE-T connections, "Cat 5e is rated for up to 55m, Cat 6a is rated for 100m, and Cat 7 is rated for 150m." Therefore, the speed issue is likely due to the fact that the connection type is not rated for the distance between the two buildings. To resolve the issue, the technician should consider using a Cat 6a or Cat 7 cable to increase the distance the connection is rated for.

A VoIP sales call with a customer would be most affected by jitter greater than 10ms on the WAN connection. Jitter is the variation in delay of packets arriving at the destination. It can cause choppy or distorted audio quality for VoIP applications, especially over WAN links that have limited bandwidth and high latency. The recommended jitter for VoIP is less than 10ms. Reference: https://www.voip-info.org/voip-jitter/

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.
Reference:
1: CompTIA Network+ N10-008 Study Guide, Chapter 3: Network Components, Section 3.1: Network Devices, p. 125
2: CompTIA Network+ N10-008 Study Guide, Chapter 3: Network Components, Section 3.1: Network Devices, p. 126
3: CompTIA Network+ N10-008 Study Guide, Chapter 3: Network Components, Section 3.1: Network Devices, p. 128
4: CompTIA Network+ N10-008 Study Guide, Chapter 6: Network Security, Section 6.3: Network Security Devices and Technologies, p. 342

The type of site that the company should implement for non-critical applications that can tolerate a short period of downtime is a warm site. A warm site is a disaster recovery site that has some pre-installed equipment and software, but not as much as a hot site, which is fully operational and ready to take over the primary site's functions in case of a disaster. A warm site requires some time and effort to activate and synchronize with the primary site, but not as much as a cold site, which has no equipment or software installed and requires a lot of configuration and testing. A passive site is not a common term for a disaster recovery site, but it could refer to a site that only receives backups from the primary site and does not actively participate in the network operations. Reference: CompTIA Network+ N10-008 Certification Study Guide, page 347; The Official CompTIA Network+ Student Guide (Exam N10-008), page 13-10.