For a number 8 wire or heavier gauge the lug would have to be marked with the words "solid" in order to use a solid wire. 

For our breakers we specify solid and stranded for wires 30 AWG to 10 AWG. 8 AWG and heavier should use stranded wire. If it is a Siemens breaker it will have to be stranded only for 8 AWG and heavier unless the breaker stipulates that solid wire is acceptable.