Twin roll casting squeezes molten metal into a thin solid sheet directly between two rotating rolls, so the terms tied to it should describe what is physically happening right at that roll gap. The mushy zone tells you where liquid and solid coexist inside the gap, deformation refers to the rolls actually working the partly solid strip, and separating force is the load the rolls feel and must be controlled to hold the right thickness. Coring, on the other hand, is simply the composition gradient that forms inside a dendrite when solidification happens slowly enough for solute to pile up, and it is a general casting microstructure feature rather than something specific to the roll gap mechanics of twin roll casting. So coring is the term that does not belong here, option (D).