For a new global ISP to connect from central offices in North America to the United Kingdom, the best cabling solution would be single-mode fiber optic cable. Single-mode fiber optic cable is a type of cable that is used to transmit data over long distances using light signals. It is typically used in long-haul communication networks, such as those that connect different countries or continents.

A pinout is the arrangement of wires or pins in a cable or connector that determines how the signals are transmitted and received. A pinout can be either straight-through, which means that the wires or pins are connected in the same order on both ends of the cable or connector, or crossover, which means that the wires or pins are swapped on one end of the cable or connector.
An incorrect pinout is a likely cause of the low data rate and port errors, because it can prevent the proper communication between the workstation and the switch. If the pinout is not compatible with the devices or the standards, the signals may be distorted, lost, or misinterpreted, resulting in errors and reduced performance. For example, if the workstation and the switch use different Ethernet standards, such as 10BASE-T and 1000BASE-T, they may require different pinouts to operate correctly. If the cable or connector has a mismatched pinout, the workstation and the switch may not be able to negotiate the optimal speed and duplex settings, and may fall back to a lower data rate, such as 100 full duplex, or experience many RX and TX errors.
Therefore, option D is the most likely answer, as the incorrect pinout needs to be corrected to provide the optimal data rate and port performance. The technician needs to check the cable and connector types, standards, and pinouts, and replace them with the appropriate ones that match the workstation and the switch specifications.
Option A is not a likely answer, as the bad switch ports would affect the connectivity and performance of any device that is connected to them, not just the workstation. If the technician moves the computer to another switchport and experiences the same issues, it indicates that the problem is not with the switch ports, but with the workstation, the cable, or the connector.
Option B is also not a likely answer, as the duplex issues would affect the mode of data transmission between the workstation and the switch, not the data rate or the port errors. The duplex issues can be either half-duplex, which means that data can be transmitted in one direction at a time, or full-duplex, which means that data can be transmitted in both directions simultaneously. The duplex issues can cause collisions or interference if the workstation and the switch have different duplex settings, but they would not cause the data rate to drop to 100 full duplex, or cause many RX and TX errors.
Option C is also not a likely answer, as the cable length would affect the signal strength and quality of the data transmission, not the data rate or the port errors. The cable length can cause attenuation, which is the loss of signal power over distance, or crosstalk, which is the interference from adjacent wires or cables. The cable length can cause the data transmission to be degraded or corrupted, but it would not cause the data rate to drop to 100 full duplex, or cause many RX and TX errors.
Reference:
CompTIA Network+ N10-008 Study Guide, Chapter 2: Network Devices and Technologies, Section 2.1: Network Cabling and Connectors, Page 661 Professor Messer's CompTIA N10-008 Network+ Course Notes, Section 2.1: Network Cabling and Connectors, Page 92 What is a Pinout? | Definition and Examples3 Ethernet Cable Pinouts - Cisco Troubleshooting Ethernet Cable Problems - Cisco

Before taking any further action, it is important to verify that the problem reported by the users is the same as the one that prompted the firmware upgrade. It is possible that the firmware upgrade did resolve the original issue, but a new or different issue has arisen with similar symptoms. By gathering additional information from the users, such as error messages, screenshots, logs, or network traces, the technician can confirm or rule out this possibility and avoid wasting time and resources on unnecessary steps.
Reopening the service ticket, requesting a new maintenance window, and rolling back to the anterior firmware version (A) is a possible option if the firmware upgrade did not resolve the original issue and caused more problems. However, this should not be done without first verifying that the users' concerns are related to the firmware upgrade and not a different issue.
Employing a divide-and-conquer troubleshooting methodology by engaging the firewall vendor's support is another possible option if the technician needs assistance from the vendor to diagnose or resolve the issue. However, this should also not be done without first gathering additional information from the users to narrow down the scope of the problem and provide relevant details to the vendor.
Escalating the issue to the IT management team in order to negotiate a new SLA with the user's manager (D) is not a relevant option at this stage. An SLA (Service Level Agreement) is a contract that defines the expectations and responsibilities of both parties in terms of service quality, availability, performance, and response time. Negotiating a new SLA does not address the root cause of the issue or help to resolve it. Moreover, escalating an issue to management should only be done when all other options have been exhausted or when there is a significant impact or risk to the business.

Network troubleshooting is a repeatable process, which means that you can break it down into clear steps that anyone can follow.
Identify the Problem. ...
Develop a Theory. ...
Test the Theory. ...
Plan of Action. ...
Implement the Solution. ...
Verify System Functionality. ...
Document the Issue.
Theory of probable cause is before Plan of action.
https://www.comptia.org/content/guides/a-guide-to-network-troubleshooting

The reason the network is not functional is that STP (Spanning Tree Protocol) is not configured on the switches. STP is a protocol that prevents loops in a network topology by blocking redundant links between switches. If STP is not enabled, the switches will forward broadcast frames endlessly, creating a broadcast storm that consumes network resources and disrupts communication. Reference: CompTIA Network+ N10-008 Certification Study Guide, page 67; The Official CompTIA Network+ Student Guide (Exam N10-008), page 2-14.