Low-beam
Low, definitely preferably fog lights. Have you ever noticed where in relation to normal headlights fog lights are placed? Below them..
Low beam. Do you want me to double as you when you take the driving test too? Read the book and get a brain so you know how to drive and not get your license revoked within 6 months.
That depends on how low the fog is. If it's ground fog, use the low beam; if it's a high fog use high beam. Driving tip in fog: Wear your sun glasses, you'll be surprized how they cut thru the fog.