This scene at the Grand Canyon has two erosional surfaces along either side of the very top set of rock layers. These erosion surfaces are marked with blue lines in the photograph below. Which geologic law indicates that these rock layers used to be wider across (from left to right), and are now narrower due to erosion

a. The Law of Lateral Continuity

Explanation:

The law of lateral continuity is used to explain that the rock layers of the Grand Canyon and indeed other layers of sediment originally used to be wider across and extended continually and laterally across the landscape.

They are now narrower now however, due to the millions of years of erosion that affected them such that now they are separated by various features such as valleys that were caused by the aforementioned erosion.