- both cost-per-click (CPC) and cost-per-acquisition (CPA) ads entering the auction
- only cost-per-click (CPC) ads entering the auction
- only cost-per-thousand impressions (CPM) ads entering the auction
- any cost-per-click (CPC) or cost-per-acquisition (CPA) ads entering the auction
The correct answer is: any cost-per-click (CPC) or cost-per-acquisition (CPA) ads entering the auction
Explanation: Actually the given answers are creating confusion. A definition of ecpm is created if CPC and vCPM ads compete for the same Display Network Placement. But, in the given answers, vCPM is missing and CPA is being questioned instead.
“To keep things fair, when CPC and vCPM ads compete for the same Display Network placement, the two types of ads are compared apples-to-apples on how much they’re effectively willing to pay for the impression. With a vCPM ad, the max viewable CPM bid represents how much the advertiser is willing to pay for each 1,000 viewable impressions; with a CPC ad, Google estimates how many clicks the ad might receive in 1,000 impressions to get the comparison.”
Read more here: https://support.google.com/google-ads/answer/2630842
This answer is wrong but possibly because Google have updated the question to have (vCPM) instead of (CPM) in the last answer.
The answer here should absolutely be when CPC and CPM ads are both entering the auction.