The correct answer is - C and D.
The principle of original horizontality means that the rock layers have been order by age, with the layers at the top being the youngest, while the layers at the bottom being the oldest.
In this picture it is little hard to tell which rocks belong to the same layer from first look because there are multiple twists and turns, an uneven surface, caused by pressure on the crust.
By carefully examining the image though, we can trace the layers more properly and see which ones are younger, which older, and which have the same relative age. In this case, we can see that the markings C and D are actually on the same layer of rocks, thus being a good indication that they have the same relative age.
