The law of original horizontality explains:

why rocks harden

why rocks appear in layers

why rock layers are always flat

how sedimentary rock forms

Answer: why rocks appear in layers

Explanation:

The principle or the law of original horizontally states that the layers of sediments are deposited horizontally in the form of layers under the action of gravity. The strata of rocks and soils being added later remains on the top whereas the older ones remains at the bottom.

On the basis of the above explanation, why rocks appear in layers is the correct option.

How sedimentary rocks form

Explanation:

More appropriately, why sedimentary rocks are layered.

It is generally believed that sediments are laid horizontally in basins of deposition and are placed on top of one another in form of layers or strata.