Explain, in terms of particle arrangement, why a sample of solid nacl has less entropy
than a sample of aqueous nacl.

In solid nacl a crystalline arrangement is found in the salt where na and cl make an ionic lattice which provides stability and reduces its entropy. 
in aqueous the salt dissolves and ions are formed which are free to move because of which they have increased entropy.

D.na is the answer hope this

Anions are negative charged ions