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

By state of matter I assume you mean solid, liquid, gas, in which case the answer would be C.
the behaviour of molecules:
- in a solid is low energy with limited activity where molecules only vibrate, cannot mould to shape of container
- in a liquid is with moderate energy, with molecules flowing over the top of one another, and thus can mould to the shape of the container
- in a gas is high energy with much activity, molecules "fly" around the container they are in and can be compressed