Which of the following is NOT a main option for selecting ad duration?

In the context of selecting ad duration on Facebook, options such as custom duration and pre-populated duration are commonly available. Custom duration allows advertisers to specify exactly how long they want their ad to run, providing a tailored approach to campaign management. Pre-populated duration typically offers default timeframes for ease of use, making it quicker for advertisers to set up their ads.

Flexible duration, while not as commonly referenced, suggests the ability to adjust the duration based on specific criteria or goals. However, standard duration is not recognized as a main option within Facebook's ad setup. While Facebook does provide ways for advertisers to choose their ad timelines, "standard duration" does not equate to an official category like the others mentioned. Thus, this lack of recognition makes it the correct response in relation to the other named options.