Which agent of erosion carried sediment that scratched bedrock

The agent of erosion, which carried sediments that scratched bedrock is A MOVING GLACIER.
Agents of erosion refers to those forces that cause erosion. There are basically four agents of erosion, these are wind, running water, glaciers and gravity.
Glaciers are large sheet or block of ice that move along a landscape. Glaciers are powerful agents of erosion; moving glaciers are capable of removing loose rocks on their way of movement. They can also pick up and move particles of varying size. Moving glaciers usually carry sediments which scratch  and polish bedrocks surfaces.

The answer is observations and interpreting.

When you are making a guess you are forming a hypothesis