Final answer:
An impression is counted after the creative is served and viewed on the site.
Step-by-step explanation:
In reporting, an impression is counted after the creative is served and viewed on the site. This means that the creative needs to be seen by a user in order to count as an impression. So option 1) After the creative is served and viewed on the site is correct.
An impression is a metric used to quantify the number of digital views or engagements of a piece of content, usually an advertisement, digital post, or web page. Impressions are also referred to as an "ad view." They are used in online advertising, which often pays on a per-impression basis.
Impressions are the number of times your content is displayed, no matter if it was clicked or not. Think of reach as the number of unique people who see your content.
However, impressions are the total number of times your content appears in news feeds. So, if one person sees your post 3 times, this counts as 3 impressions, even though you've only reached one person.