What filter does a Dreo HM311S take?
None — this model has no replaceable wick or filter.
Dreo HM311S, ultrasonic humidifier, tabletop.
Filter-free as sold. Dreo offers an optional 311-series demineralization cartridge for this family (not yet priced on this site).
This entry exists so the answer is complete: if a store tries to sell you a replacement filter for this model, it is not a manufacturer part.