Which determines the state of matter for any material?
a. the size of the atoms
b. the hardness of the material
c. the behavior of the molecules
d. the weight of the material

I think the answer is c) the behaviour of molecules is the closest.
i learnt that particle movement and attraction within matter determine whether it is solid, liquid, or gas.
so if it's gas, the particles within it are spread apart and are moving randomly, whereas in a solid, they have strong forces of attraction between the particles and so they stay in a rigid lattice :)