The correct answer is - the first, black layer.
If the situation is that the rock layers have not been disturbed, and their alignment is in order as the periods in which they have been on the top, before being covered with new sediments, than the oldest fossils will be at the bottom layers, will the newest in the top layers.
In this situation, we know that the mollusks have evolved before the arthropods, and since the arthropods are found in the second, grey layer, the mollusks will be found in the layer bellow them, the first, black, layer.
