Answer:
0.600 moles has volume 1.200L
Step-by-step explanation:
The volume is numerically double the moles
0.600 mol occupies 1.200L 
Mathematically: 
 V1/moles1 = V2/moles2 
1.200L / moles1 = 0.600L/ 0.300mol 
Moles 0.600L = 1.200L * 0.300mol 
Moles = 1.2000L *0.300 mol / 0.600L 
moles = 0.600 moles.