A, the atomic radius of the same main group increases from top to bottom, so the atomic radius: f < cl < br < I, so A is wrong;

B, the stronger the nonmetal, the more stable the gaseous hydride, nonmetal O > Cl > Se, and the hydride stability is H2O > HCl > H2S, so B is wrong;

C, the stronger the metallicity, the stronger the alkalinity of the hydrate of the highest valence oxide, and the order of metallicity is Na > Mg > Al, then the alkalinity is NaOH > Mg (OH) 2 > Al (OH) 3, so C is correct;

D, the stronger the nonmetal of the element, the stronger the acidity of the hydrate corresponding to the highest valence oxide. The nonmetal is Cl > S > P, and the acidity is H3PO4 < H2SO4 < HCLO4, so D is wrong;

So choose C.