A direct response advertiser is primarily interested in generating conversions from a display campaign. To get a potentially better return on investment, this advertiser should use:
viewable cost-per-thousand-impressions (vCPM) bidding
target cost-per-acquisition (CPA) bidding
cost-per-day (CPD) bidding
cost-per-click (CPC) bidding
Explanation: An automated bid strategy that sets bids to help get as many conversions as possible at the target cost-per-acquisition (CPA) you set.
Note: Conversion Optimizer is now Target CPA bidding
Read more here: https://support.google.com/adwords/answer/2390684?hl=en