Final answer:
The ITX form factor, especially Mini-ITX, is the motherboard form factor with the smallest footprint suitable for thin client devices due to its compact size.
Step-by-step explanation:
The motherboard form factor that has the smallest footprint for use in thin client devices among the given options is ITX. Specifically Mini-ITX is often used for these purposes due to its compact dimensions of 17cm x 17cm which is much smaller than a standard ATX or even Micro-ATX motherboards.
Although Mini-ITX was not one of the choices listed, it is a subset of the ITX form factor and is known for its small size and is suitable for thin clients indicating that the ITX form factor in general is the correct answer.