Which user characteristic may NOT be used to change keyword bids in AdWords?:
time of day
The correct answer is:
It comes under the bid adjustment. Bid adjustments allow you to show your ads more or less frequently based on where, when, and how people search. For example, sometimes a click is worth more to you if it comes from a smartphone, at a certain time of day, or from a specific location.
Read more here: https://support.google.com/adwords/answer/2732132