Final answer:
A nappe is a folded, allochthonous rock unit that has moved on a predominantly horizontal surface, commonly found in regions of significant crustal compression like the Alps.
Step-by-step explanation:
A nappe is defined as a folded, allochthonous rock unit that has moved over a predominantly horizontal surface. This geological feature is typically associated with regions that have undergone significant compression during orogeny, leading to the thrusting and emplacement of large slices of crust over each other. An example of a nappe can be seen in the Alps, where European and African tectonic plates have moved and created complex fold structures. It is not related to magma mixing, partial melting, or being an open fold; hence, the correct answer is b. folded, allochthonous rock unit, which has moved on a predominantly horizontal surface.