Ceramics are traditional inorganic nonmetallic materials, and Al _ 2Si _ 2O _ 5 (OH) _ 4, the main component of clay, is one of the raw materials for preparing ceramics. The following is about Al.

Answer: A. According to the main component Al _ 2Si _ 2O _ 5 (OH) _ 4 of clay, we know that it is made of? Elements of aluminum, silicon, oxygen and hydrogen; Therefore, A is correct;

B, according to the algebraic sum of positive and negative valence in the compound is zero, and the valence of Al in clay is 2x+(+4) × 2+(-2) × 5+(-1) × 2 = 0, then x =+3; Therefore, B is correct;

C, according to the mass ratio of each element in the compound = relative atomic mass of each element × atomic number, it is concluded that the mass ratio of hydrogen and oxygen in clay is 4: 16× 9 = 1: 36, so C is wrong;

D, according to the mass fraction of an element in the substance = × 100%,

The mass fraction of silicon in clay = × 100%=2 1.7%, so d is wrong;

So choose ab.