Answer : Th correct option is, a copper-gold alloy
Explanation :
Solid : It is defined as that form of matter which possesses rigidity and hence possesses a definite shape and a definite volume.
All the solid elements and compounds exist in the crystalline form.
Crystalline structure : It is defined as when the constitute particles are arranged in a definite geometric pattern inn the three dimensional space so theta there is short range as well as long range order of particles. For example all the elements and the compounds.
From the given options, a copper-gold alloy has a crystalline structure because it is a solid. While the sugar water, salt water are the liquids and air is a gas.
Hence, the a copper-gold alloy has a crystalline structure.