If the sequence is undisturbed which rock layer is the oldest? which is the youngest? explain your answer

Looking at a basic timeline on wikipedia, I think the oyster fossils because invertebrates evolved before vertebrates and fish came before land plants. Although ferns are relatively early landplants, they are not the first land plants. Therefore, based on wikipedia, the oyster fossils.

However, limestone is a little curious because limestone usually results from shells of ancient animals. I don't know if that's a red herring,

They both determine the exact age of rocks.

The answer would be d. pili