Answer:
C. Directional selection
Step-by-step explanation:
The graph given shows that an extreme phenotype is favoured as allele frequency shifts towards the direction of the phenotype. Thus, due to the natural selection that occured as a result of draught, an extreme trait which is a particular beak size of the birds is favoured over others.
One phenotype is selected over the other in one direction. This is an example of directional selection, which occurs when one trait at the end of a spectrum is selected for against the others that have less fitness.