Right answer is (a) the standard k-ε model and the k-ω model
For explanation I would say: For boundary layers with adverse pressure gradients, the standard k-ε model does not perform well. To overcome this problem, the other models were devised. But, those are sensitive to the arbitrary constants used. So, the Shear Stress Transport model combines the standard k-ε model and the k-ω model to get the advantages of both.