Consider a species of group-living bird in which individuals produce alarm calls to alert group members of the presence of a predator. The alarm call confers fitness benefits to the caller as it helps group members (composed of genetic relatives) escape the predator. However, alarm calling also makes the caller more conspicuous to the predator. Individuals of this species in a population have four phenotypes for the loudness of alarm calls they produce in the order P < Q < R < S. The graph below gives the cost and benefit functions for alarm calling behaviour for the four phenotypes.

Which one of the following phenotype frequencies represents the correct outcome of natural selection?