In eukaryotic flagella, the fibers that slide past one another due to the activity of dynein proteins are microtubules.
Microtubules are one of the three main components of the cytoskeleton, alongside microfilaments and intermediate filaments. In the structure of eukaryotic flagella and cilia, microtubules are arranged in a characteristic ‘9+2’ pattern, where nine pairs of microtubules surround two central microtubules.
Dynein is a motor protein that moves along these microtubules and is responsible for the sliding motion that generates the bending movements of the flagella. This sliding allows the flagella and cilia to beat effectively, propelling the cell through liquid environments. Therefore, it is clear that in this context, microtubules are the correct answer.