Amongst all the three phases, it is solid phase which is the most compact because the atoms within the solids are bound with a very strong force of attraction. But, solids can exist as either crystalline solid or amorphous solid. Between these two type of solids, it is crystalline solids which are very well packed and the inter atomic distance between their two atoms is the least. It is due to the reason that the atoms within a crystalline solid are arranged in proper order in the form of stacks. But, in amorphous solids the atoms are not arranged properly i.e. their arrangement is random which leads to the increased inter atomic distance. Second comes liquid phase, in liquids the inter atomic forces are intermediate between solids and gases. Atoms in a gas are bound with the weakest force of attraction known as van der Waals that is why the distance between the atoms of a gas is the highest.
I am pretty sure that for Most Substances the distance between particles is smallest when the substance c.exists as a crystalline solid. I pick this option because crystalline solid is the most compact substances unlike the others. It will definitely help you.
