Which of the following will be used in smaller form factor servers to allow an additional network interface card to be inserted parallel to the motherboard?

Ribbon cables are used for internal data connections, not for mounting expansion cards.

A riser card is a circuit board that plugs into a motherboard's expansion slot (e.g., PCIe) and provides additional slots, often at a right angle or parallel to the motherboard, enabling the installation of expansion cards in space-constrained cases.

A SATA controller manages storage devices and is not used to allow expansion cards to be inserted parallel to the motherboard.

A PCIe slot is where an expansion card connects, but it does not inherently allow for parallel insertion; a riser is needed for that specific orientation.