Final answer:
A selective sweep leads to reduced genetic variation around an advantageous allele. Therefore, the correct answer is 'Low genetic diversity surrounding the allele.'
Step-by-step explanation:
A selective sweep is a scenario in biology where an advantageous allele quickly spreads through a population due to natural selection. When such a sweep occurs, genetic variation around the allele of interest is reduced because the advantageous allele becomes fixed in the population and the nearby regions, which are inherited together with the advantageous allele, also become more uniform due to linkage disequilibrium. Therefore, the evidence that an allele has experienced a selective sweep in the recent past would be b. Low genetic diversity surrounding the allele, since the sweep would have purged nearby variations.