Final answer:
The smallest motherboard form factor among the given options is FlexATX, which, due to its dimensions, allows for more compact case designs.
Step-by-step explanation:
The smallest motherboard form factor among the options provided is FlexATX. The FlexATX form factor is a derivative of the ATX standard which was designed to be more flexible and smaller in size compared to traditional ATX form factors. It measures 229 x 191 mm, allowing for smaller case designs. MicroATX is slightly larger than FlexATX at 244 x 244 mm, and MicroBTX varies in size but is generally larger than FlexATX. The NLX form factor is older and not common in modern builds; its size is also larger than that of the FlexATX.