The concept in use when dual power supplies are connected to different power sources is fault tolerance. Fault tolerance is the ability of a system to continue operating without interruption or loss of data in the event of a failure of one or more components. By connecting dual power supplies to different power sources, the system can switch to the alternative power supply or source if one fails, ensuring continuous availability and reliability.
References: CompTIA Server+ SK0-005 Certification Study Guide, Chapter 1, Lesson 1.3, Objective 1.3

NIC stands for network interface card, which is a hardware component that allows a server to connect to a network. Configuring NICs in pairs on a server would provide network redundancy, meaning that if one NIC fails, the other one can take over and maintain network connectivity. The other options are not related to network redundancy.

A firewall is a network device or software that filters and controls the incoming and outgoing traffic based on predefined rules. A firewall can block or allow certain types of packets, ports, protocols, or IP addresses. The output of the ping command shows that the local host is unreachable, which means that there is no network connectivity between the source and the destination. This could be caused by a firewall that is blocking the ICMP (Internet Control Message Protocol) packets that ping uses to test the connectivity. References:
https://www.comptia.org/training/resources/exam-objectives/comptia-server-sk0-005-exam-objectives (Objective 2.2)