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 happens to the support structure when the infill is
100%?
the support structure becomes strong
the support structure becomes hollow
the support structure becomes brittle

Explanation:

The text states that using 100% infill makes the object very brittle. Since the support structure is part of the object, it would also become brittle.

Answer:

the support structure becomes brittle