Flattening of matter in the protoplanetary disk is considered a result of which physical process?

Correct answer: Rotation and gravitational collapse

The direct answer is C: rotation and gravitational collapse. A protoplanetary disk begins as a large cloud of gas and dust. Gravity pulls the material inward, so the cloud collapses and becomes smaller. At the same time, the cloud rotates. As it contracts, conservation of angular momentum makes the rotation faster, and collisions and motion tend to spread material into a flattened rotating disk around the young Sun. Therefore C is correct: both gravitational collapse and rotation are important parts of the process. Option A, soil erosion and deposition, occurs on planetary surfaces and does not form a protoplanetary disk. Option B, phases of the Moon, are caused by the changing portion of the Moon lit by the Sun as seen from Earth; they do not flatten a cloud of cosmic matter. Option D, ocean tides and ebbs, are caused mainly by gravitational effects of the Moon and Sun on oceans and are not the process that creates the early disk. The reasoning chain is: gas and dust cloud → gravity causes collapse → rotation increases during contraction → material forms a flattened disk. Memory cue: “collapse pulls inward; rotation spreads into a disk.”