Answer:
The Production Possibility Curve is concave because marginal rate of transformation, which is the slope of the curve, increases continuously as each additional unit of one good is produced by reducing quantity of the other good. It means to produce more and more units of one good, more and more units of other good need to be sacrificed. MRT and opportunity cost increases because no resource is equally efficient in production of all goods. This can be explained with the help of the following diagram. 
In the above figure, AE represents the PPC for capital good, and consumer goods. Suppose that the initial production point is B, where 1 unit of capital good and 48 units of consumer goods are produced. To produce one additional unit of capital good, 4 units of consumer good must be sacrificed (point Q Thus, at point C, the opportunity cost of one additional capital good is 4 units of consumer goods. Further moving to point D, to produce one more unit of capital good, 9 units of consumer goods must be sacrificed. That is, the opportunity cost rises to 9 units of consumer goods. Thus, as we move down the PPC from point C to point D, the opportunity cost and MRT increase. This confirms the concave shape of the PPC.
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