In order to use target cost-per-acquisition (CPA) bidding, an advertiser must:

• be opted in to the Search Network
• be using viewable cost-per-thousand impression (vCPM) bidding, with a certain number of viewable impressions in the previous 30 days
• be opted in to the Search Network and YouTube
• have a certain number of conversions in the previous 30 days

The correct answer is: have a certain number of conversions in the previous 30 days

Explanation: Before you can set up a Target CPA bid strategy, your campaign will typically need to meet the following requirements:

• You’re already tracking conversion data in your AdWords account. You can do so by either setting up conversion tracking or cross-account conversion tracking (MCC accounts), or by importing your goals and transactions data from Google Analytics.
• The ad group or campaign has received 15 conversions in the last 30 days. This conversion history enables the system to make accurate predictions about your future conversion rate. So, the more data we have, the more accurate we can be.
• The ad group or campaign has received conversions at a similar rate for at least a few days.

Read more here: https://support.google.com/google-ads/answer/6268632