Which motherboard form factor is smaller than ATX but retains many of its features?

The correct answer is microATX. This form factor is designed to be smaller than the standard ATX while still offering many of the same features, such as support for up to four expansion slots, similar I/O capabilities, and compatibility with a range of ATX accessories. This balance makes microATX a popular choice for users looking to build compact systems without sacrificing functionality.

In contrast, Mini-ITX is significantly smaller than microATX and typically supports fewer expansion slots (usually just one), making it less versatile in terms of add-on cards and peripherals. Therefore, while it is smaller, it does not retain as many of the features of ATX or microATX.

Mini-ATX is not a recognized standard and may confuse users, as it is not widely used or defined in the same way as the other options. Advanced Technology eXtended (ATX) is, by definition, larger than microATX, making it irrelevant in this context.

Thus, microATX presents an effective compromise between size and feature set, which is why it is the correct choice.