Organic matter (humus) is generally less dense than the mineral particles of the soil, so it tends to float on the surface of the water during sedimentation. This makes the statement correct. Therefore, the correct answer is decaying organic matter remains afloat - option A.

B. Largest and medium size particles settle at the bottom: The largest (sand, gravel) particles settle at the bottom first because they are the heaviest and largest, but the medium (silt) and smallest (clay) particles settle in subsequent layers above the largest particles or remain suspended for longer, not all at the very bottom.

C. Remains of decaying organic matter settles at the bottom: As explained above, decaying organic matter is less dense than mineral soil particles and thus floats, it does not settle at the bottom.

D. Largest soil particles remain suspended in water: The largest soil particles (sand/gravel) are the first to settle to the bottom due to their greater size and weight, while the smallest particles (clay/silt) remain suspended longer.