To determine the largest neutral atom you can build using the "Build an Atom" simulation, you should focus on the atomic structure, which consists of protons, neutrons, and electrons.
The largest neutral atom listed in your options is neon (Ne). Neon has an atomic number of 10, which means it has 10 protons and, in a neutral atom, also 10 electrons.
While the other elements listed (sodium, fluorine, magnesium) have higher atomic numbers, they are not larger or do not fit the criterion of neutrality when considering the configurations typically used in such simulations.
Thus, based on your options, the largest neutral atom you can build is:
neon (Ne).