The correct answer is B. Shredders.
Shredders are a type of functional feeding group that are involved in the process of in-stream physical breakdown of coarse material into fine particulate organic matter. They do this by breaking down large pieces of organic matter, such as leaves, twigs, and branches, into smaller pieces that can be consumed by other organisms.
Collector filterers are a type of functional feeding group that filter small particles of food from the water. They do this by using their specialized mouthparts to create a current that draws food particles into their mouths.
Grazers are a type of functional feeding group that graze on algae and other small plants. They do this by using their mouthparts to scrape algae and other plants off of rocks and other surfaces.
Collector gatherers are a type of functional feeding group that gather food particles from the substrate. They do this by using their mouthparts to pick up food particles from the substrate.
Here is a diagram that shows the different functional feeding groups in a stream ecosystem:
The diagram shows that shredders are the only functional feeding group that is involved in the process of in-stream physical breakdown of coarse material into fine particulate organic matter.