The option that best describes the physical weathering of a rock by water is: water breaks down a rock into smaller pieces with the same composition as the larger rock.
Physical weathering, also known as mechanical weathering, involves the physical fragmentation of rocks into smaller pieces without changing their chemical composition. In this process, water can contribute by freezing and thawing cycles (ice wedging), or by flowing over rocks and causing abrasion, but it does not dissolve the rock or change its chemical structure.