What must be installed to accommodate an additional PCIe NIC in a server with fully populated slots?

To accommodate an additional PCIe Network Interface Card (NIC) in a server that has fully populated slots, installing a riser card is the appropriate solution. A riser card provides additional PCIe slots, allowing for more expansion cards to be added to a server without requiring replacements or removals of existing cards. This is particularly useful in compact server designs where space is limited, and existing expansion slots are already in use.

Riser cards effectively extend the motherboard's PCIe slots, facilitating the installation of additional hardware such as NICs while maintaining a manageable footprint within the server chassis. This capability is crucial for servers that need to expand their networking capabilities or accommodate other peripheral devices.

The other options, such as installing a virtual NIC, do not apply here as they deal with software-based networking rather than physical hardware expansions. A daughtercard typically refers to a smaller expansion card that connects to a main circuit board but would not resolve the limitation of fully populated PCIe slots in the way a riser card does. Meanwhile, a KVM (Keyboard, Video, Mouse) switch is unrelated to the installation of networking hardware, as it is used for managing multiple servers from a single set of peripherals, not for adding new NICs.