Question

Which layer in Figure 4-1 forms the cap rock for the oil trap?

Answer 1

I am pretty sure that the layer in figure 4-1 which forms the cap rock for the oil trap is the layer  F that shows an example of stratigraphic traps that includes unconformity, lens, pinch-out in which you can see how the oil is stuck inside the porosity of the rocks. 
Do hope you will find it helpful! Regards.