Question

fff: types of infill
use the information below to answer the question about types of infill.
when you are designing an object with fff, you will need to decide what
percent of material and space will make up the infill. if you are wanting a very
strong object with little to no movement, you will want it to be a 50% - 60%
infill. when you start working on a design the default is 20% - 25% which is
normally a good percent of infill for your project. you will probably never want
to use 100% infill, because that you make your object very brittle, and increases
the risk of warpage.
what type of structure has a default setting of 20% when
using the fff modeling process?
infill
horizontal bridge
fillet

Explanation:

The text clearly states that when starting a design with FFF, the default for the structure that determines the percent of material and space (infill) is 20% - 25%. The other options (horizontal bridge and fillet) are not mentioned in the context of default settings related to the described percentage - based structure in the FFF modeling process.

Answer:

infill