Answer:
It is explained in the question itself that the resources are not specialized for the production of any one commodity. They have mid that the same resource can be used for the production of both smart phones as well as the tablets. Thus, when the UK increases the production of the smart phones, the opportunity cost of producing the Tablets remains constant or remains the same because the trade-off between the goods will be constant.
Thus, when the opportunity cost remains constant and the economy carries on the production, then the Production Possibility Frontier of the economy will be represented with the help of a straight line PPF. Here in the graphs, Graph 1 represents the straight line PPF and thus, the trade-off between the smart phones and the tablets will be represented by the Graph 1.