We need to identify which of the groups of respondents would most likely call into question the results of the survey.
That group corresponds to the one that corresponds to a biased sample. In a biased sample for this survey, the respondents have a higher probability of selecting one brand over the other as their favorite.
Notice that blinded people or people that are chosen randomly are not biased.
On the other hand, people related to the employees of one company have a higher probability of choosing their company, thus representing a biased sample.
Therefore, the answer is:
Answer