Which of the following processes contributes to the mechanical breakdown of rocks without changing their composition?

The correct choice is physical weathering, which refers to the mechanical breakdown of rocks into smaller fragments without altering their chemical composition. This process can occur through various means, such as temperature fluctuations leading to freeze-thaw cycles, abrasion by wind or water, and root expansion from plants.

Physical weathering effectively reduces the size of rock particles, making them more susceptible to other weathering processes, yet it does not modify the minerals present in the rocks themselves. This characteristic is crucial because it means that the original material remains intact while undergoing physical change in shape and size.

In contrast, chemical weathering involves chemical reactions that can dissolve or alter the minerals in rocks, while biological weathering includes processes where living organisms, like plants and lichens, contribute to breaking down rocks, potentially affecting their composition. Biogeochemical weathering combines aspects of physical and chemical weathering, indicating interactions between biological, geological, and chemical processes, but still does not focus solely on mechanical breakdown without changing composition.