Explanation
Near field communication (NFC) is a function that enables contactless payments with a phone by allowing it to communicate with compatible devices within close proximity. If NFC is not working properly, the user will not be able to perform contactless payments with the phone. Hotspot is a feature that allows a device to share its internet connection with other devices via Wi-Fi, Bluetooth, or USB. Bluetooth is a wireless technology that allows devices to connect and exchange data over short distances. USB is a standard interface that connects devices such as keyboards, mice, flash drives, etc

Explanation
Device interference is the most likely cause of the wireless connection dropping or becoming slow.
Interference can come from other wireless devices, microwave ovens, or physical barriers like walls and metal objects. Hidden SSID, Ethernet port flapping, and high ISP latency are not likely to cause this issue.
Wireless interference from other devices can cause a drop in wireless connection speed and stability. In a small office environment, there may be several wireless devices competing for the same bandwidth, causing the issues described. References: CompTIA A+ Certification Exam Core 1 Exam Objectives - 3.7 Given a scenario, troubleshoot wired and wireless networks with appropriate tools

Explanation
The most likely cause of the wireless network connectivity issues is A. External interference.
External interference is any signal or noise that disrupts or degrades the quality of a wireless network. External interference can come from various sources, such as other wireless networks, cordless phones, microwaves, Bluetooth devices, or electrical appliances. External interference can affect the wireless network performance, reliability, and security, by causing issues such as signal loss, packet drops, latency, or interference.
In this scenario, a company in a large building uses a wireless network for its workstations. After a new company moves into the building on the floor below, the old company's wireless network begins experiencing connectivity issues. This could indicate that the new company's wireless network is using the same or overlapping frequency channel as the old company's wireless network, and creating external interference.
Wireless networks use radio frequency channels to transmit and receive data, and each channel has a certain bandwidth and range. If two or more wireless networks use the same or overlapping channel, they may interfere with each other and cause congestion or collision.
To resolve this issue, the old company's network technician should change the wireless network channel to a different one that is less crowded or more compatible with the wireless network standard. The technician should also check the wireless network settings and features, such as SSID (Service Set Identifier), encryption, authentication, or power level, and adjust them if necessary. The technician should also scan the wireless environment and identify the sources and levels of external interference, and try to avoid or minimize them if possible.
For more information about external interference and how to prevent or reduce it, you can refer to the following web search results: [What Is Wireless Interference?], [How to Find the Best WiFi Channel for Your Router], [How to Reduce Wireless Interference].

Explanation
SMTP (Simple Mail Transfer Protocol) uses port 25 to send email messages between servers12.
SMTP uses port 25 to send email messages between mail servers. Therefore, the system administrator should enable port 25 to allow SMTP traffic through the host-based firewall. References: CompTIA A+ Certification Study Guide, Tenth Edition, pg. 282