Question

in an area where a river has cut deep into earth, there are several layers of very different rock exposed
question
the oldest rock layer is most likely to be the layer that is -
the thickest layer.
the most rich in fossils
below the other layers. (on bottom)
igneous intrusive rocks

Explanation:

According to the principle of superposition in geology, in undisturbed sedimentary rock layers, the oldest layer is at the bottom. Thickness does not determine age. Fossil richness is not a reliable indicator of age for the entire layer. Igneous intrusive rocks can be younger than surrounding sedimentary layers if they intrude into them.

Answer:

Below the other layers. (on bottom)