Smallest unit of Sodium Chloride
A “formula unit” would be closer to the answer than a “unit cell”. A unit cell is used in crystallography to define the smallest repeating unit of the crystal that has all the symmetries of the overall crystal structure. For NaCl, there are a variety of different ways to visualize the unit cell (2 interpenetrating FCC lattices, FCC with a 2-atom basis, etc. ), but regardless, there are four formula units of NaCl in the NaCl unit cell (4 Na and 4 Cl atoms).