Elements such as those on the periodic table are unable to be broken down into simpler forms.
Mixtures (both homogenous and heterogenous) are made of multiple elements making it able to be physically broken down back to its original elements.
Compounds are also made up of multiple elements, however it must go through a chemical separation process in order for it to be broken down into its original elements.
Matter is a broad term of any substance. So it could both be a combination or a single element making it incorrect for this question.
Decreasing the volume would cause the equilibrium to shift to the side with less moles on it. Because both sides have the same number of moles (2) there would be no change to the equilibrium.
