Final answer:
The frequency of the a allele at the Avirulence locus of the potato crop infected with Phytophthora infestans cannot be determined based on the given information.
Step-by-step explanation:
The question is asking about the frequency of the a allele at the Avirulence locus of the potato crop infected with Phytophthora infestans. However, no specific information about the frequency of the a allele is provided in the given reference material. Therefore, the answer to the question is option 4) Not enough information.