Final answer:
The system unit designed to be placed either horizontally or vertically is the desktop case. They provide flexibility in orientation, unlike tower cases which are typically vertical or all-in-one and rack-mounted cases that have fixed orientations.
Step-by-step explanation:
The correct answer to the question is b) Desktop case. Desktop cases are designed to be placed either horizontally or vertically. Unlike tower cases, which are typically designed to stand vertically, desktop cases offer more flexibility and can be oriented based on user preference or space constraints. All-in-one cases integrate the computer components and monitor into a single unit, and these are not meant to be reoriented. In contrast, rack-mounted cases are designed to be mounted in a rack and are not suited to being placed horizontally on a desk.