They are arranged by their atomic mass.
The correct answer is option B. The most dense phase of matter is the solid phase and the least dense are gases. However, there is an exception. Water is the exception. Solid water or ice is less dense than the liquid phase therefore it floats on liquid water.
1:2:6? Ba(NO3)2 +Na2SO4 -----> BaSO4 + 2NaNO3
Ba(NO3)2
BaSO4
NaNO 3
Na2SO4