Answer:
The correct option is : a. directional selection
Explanation:
The directional selection is the method of natural selection. A particular phenotype is favored in this selection. This results in the allele frequency to shift in a direction of that particular phenotype, over time.
Therefore, this selection leads to an increase in the preferable allele due to the differences in reproduction and survival among the different phenotype. This increase is independent of dominance of the allele. Even recessive allele, will eventually become fixed.