Which of these cuases rocks to weaken and crumble?

A tricky question!! Decomposition of organic matter contributes to the formation of soil, which makes chemicals which can weaken rocks... and glaciers can also cause MAJOR erosion, weakening and crumbling rocks quite well. But, i think the answer being sought here, is the formation of rust from the more reduced forms of iron in many rocks. When iron oxidizes in rocks, this weakens the rocks the iron is in, sometimes quite a bit. It is a definite erosional process.