The most effective way to control the number of times someone sees an ad is by:

A) Monitoring audience retention metrics with YouTube Analytics
B) Setting a lower cost-per-view (CPV) bid at the target group level
C) Lowering bidding levels
D) Setting a frequency cap

The correct answer is:

D) Setting a frequency cap

Explanation: Frequency is the average number of times a unique user sees your ad in a position of “1” over a given time period. Use frequency capping to help control the maximum number of times that you’d like each person to see your ad on the Display Network.
Read more here: https://support.google.com/adwords/answer/117579?hl=en