Which vehicle placarding requirement applies to a tank vehicle carrying Class 2.1 flammable gas (e.g. UN 1978 Propane)?

1. Class 3 placards (flame symbol on red background) on front hood
2. Class 6.1 placards (skull and crossbones) on rear bumper
3. Class 2.1 placards (flame symbol on red background) on both sides and rear
4. Class 8 placards (corrosive symbol) on side panels