Final answer:
The type of system unit designed to be placed both horizontally and vertically is a desktop computer.
Step-by-step explanation:
The type of system unit that is designed to be placed either horizontally or vertically is known as a desktop computer. Unlike the other options, which include notebook, smartphone, and tablet pc, a desktop computer can be configured in either orientation to fit the user's available space and preference. This flexibility allows for better utilization of workspace and is a feature that is typically associated with the traditional tower or mini-tower cases often used with desktop PCs.
The system unit that is designed to be placed either horizontally or vertically is A. desktop. Desktop computers have a versatile form factor that allows them to be positioned in different orientations based on the user's preferences and space availability.