According to the principle of original horizontality what most likely happened to the rock layers in the background ?

The principle of original horizontality states that all rock layers were initially deposited horizontally.  The answer must be They were deposited horizontally and then shifted by a geologic event because they other choices all indicate that the the rocks were initially deposited in a way that was not horizontal.

Hope this helps!!

D. They were deposited horizontally and then shifted by a geologic event.

Explanation: The only one that explains horizontally.

the mantle

explanation:

earth is divided into three layers: the crust or the outermost layer on which we live; the underlying, middle layer - the mantle - and the core (the innermost layer of earth).