The best way to reserve an ad is to:

A) Create a reservation campaign in AdWords
B) Contact a Google sales representative
C) Create a standard video campaign
D) Enter specifications on the “Reservation” tab

The correct answer is:

B) Contact a Google sales representative

Explanation: As an advertiser or ad agency, you can buy placements on a reservation basis-instead of through the AdWords auction–when you want to pay based on the number of impressions (also known as cost-per-thousand impressions) or per day (cost-per-day). Reservation campaigns also let you buy impressions at a fixed rate.

Read more here: https://support.google.com/partners/answer/6030919