• hovers their finger over the ad for at least 2 seconds
• watches the ad for at least 2 seconds
• watches the ad twice in a rob
• expands or interacts with the ad

The correct answer is: expands or interacts with the ad

Explanation: engagements are charged when the user engages with the ad. For expandable ad spaces, “user engagement” means that the user hovers, clicks or taps to expand and the ad has time to load and be seen. For pre-expanded ads (like interstitials), “user engagement” means that the user interacts with the ad after it displays.

Read more here: https://support.google.com/google-ads/answer/6379520