Which electric plug? Russia, Belarus,Poland ,and Scandinavia

The normal type C as shown here.

Type C should work, but it is not approved for appliance drawing over 2.5 amps. That's fine for cell phone, USB. and most computer chargers, but not for hair dryers. A modified Schuko adapter, which plugs into both Type E (Belarus, Scandinavia, Russia) and Type F (Poland, Russia) receptacles and has a grounding hole for Type E and side grounding clips for Type F, would be better, but the appliance should have a grounding pin on the US plug. Note: two pin plugs can be connected two way and are thus not "polarized". US devices with polarizing plugs (one blade wider) require for safety a polarized power supply, which is not available in most European countries (except Switzerland).