We have no reason to think the sample of items sent by the inspectors is too small. In order to pick (A), we need some textual hook in the stimulus to suggest the sample size is too small.

The assumption is that rate of choosing defects did not exceed the actual defect rate. (Ex. Actual rate is 25% defects, and inspectors chose defective items 25% of the time, nondefect 75%.) This doesn’t require chance of choosing defect to be equal to choosing nondefect (50-50).

This doesn’t undermine the argument, because we have no reason to think that a few sites being responsible for most defects would skew the defect rate in the inspectors’ sample. We have no reason to think the inspectors disproportionately picked items from these few sites.

This undermines the argument, because it shows that the sample selected by inspectors might have a higher rate of defect than the average product produced by the supplier. This is why we can’t assume the over 20% defect rate in the sample applies to the overall set of products.

The author does assume that the manner in which the inspectors collected items for the sample was not biased in a way that made the defect rate in the sample unrepresentative of the overall defect rate, but this doesn’t require any view about the number of visits.