Which of the following is NOT a size of motherboard form factor?

The correct answer, indicating that Mini-TX is not a recognized motherboard form factor, is supported by the established standard forms of motherboard sizes. Mini-ITX, Micro-ATX, and Standard ATX are all well-documented and widely used motherboard sizes in the industry.

Mini-ITX is popular for compact builds, typically measuring 6.7 x 6.7 inches. Micro-ATX is slightly larger at around 9.6 x 9.6 inches, offering more expansion slots than Mini-ITX, while Standard ATX measures 12 x 9.6 inches and provides the most expandability and features.

In contrast, Mini-TX does not exist as a standard form factor in computer building or design, making it the correct response to the question. The absence of Mini-TX in literature and practice confirms that it is not a recognized motherboard size, unlike the others mentioned which are critical in understanding computer hardware configurations.