Answer:
a. 2,000 questionnaires.
Explanation:
Winer should mail 2,000 questionnaires because according to the first mailing he got 400 completed surveys so it would be wise of him to repeat the mailing of 2,000 questionnaires because there is a probability that the completed surveys will be up to or above 1,600 to complete the initial aim of 2,000 completed surveys.