A flag would be about five hundred micro arcseconds. (about 1 meter, at around 375 Mm)

The Event Horizon Telescope has a resolution of about [20 micro arcseconds](https://eventhorizontelescope.org/).

Therefore, EHT could resolve a flag on the surface of the moon *if* it were a radio source.

However, to view it optically would require around a 350 meter aperture. The largest 'single' telescope today is around [10 meters](https://en.wikipedia.org/wiki/Gran_Telescopio_Canarias)

The Cambridge Optical Aperture Synthesis Telescope ([COAST](https://en.wikipedia.org/wiki/Cambridge_Optical_Aperture_Synthesis_Telescope)) - the largest I'm aware of - can resolve to around 1000 micro arcseconds, so wouldn't be able to see a flag.