B, according to the oxide is a compound composed of two elements, one of which is oxygen, it can be judged that tannic acid is not an oxide; Therefore, B is correct;
C, accord to that mass ratio of each element in the compound = relative atomic mas of each element × atomic number, the mass ratio of carbon, hydrogen and oxygen in the tannic acid composition is (12× 76): 52: (16× 46);
In tannic acid composition, the atomic number ratio of carbon, hydrogen and oxygen is 76: 52: 46; So, c is wrong;
D, according to the number marked in the lower right corner of the element symbol indicating the number of atoms contained in a molecule, the number of atoms contained in a tannic acid molecule can be obtained as follows:
76+52+46= 174; Therefore, D's statement is correct;
So choose C.