What arrangement of atoms or molecules determines whether a solid is crystalline or amorphous? internal external

The correct answer is internal.

A solid substance whose components like molecules, atoms, or ions are aligned in a highly ordered microscopic composition producing a crystal lattice, which extends in all the directions is known as a crystalline solid or a crystal.  

On the other hand, the noncrystalline solid substance in which the atoms and molecules are not aligned in a definite lattice pattern is known as an amorphous solid. Such solids comprise plastic, glass, and gel.