The images show sedimentary layers before and after they were shifted by a geologic event.

after the geologic event, which layer is the oldest?

layer a
layer b
layer c
layer d

Layer A is the oldest

Explanation:

The image to the left shows an undisturbed sedimentary sequence and we can use it as our type section. The layers have fossils that can be used to correlate them with other terrane. Here, layer A is at the bottom.

After the geologic event which overturns the rock layers, we see that layer A has come on top and the whole sequence has subsided.

Applying the law of superposition of strata, which states that "in an undisturbed sequence of sedimentary rocks, the oldest rock layer is at the bottom of the sequence".

Using this law, we see that layer A must have formed first D being the youngest.