Test and analysis: a. The sum of relative atomic masses of carbon atoms in tannin is 12×72=864, that of hydrogen atoms is 1×52=52, that of oxygen atoms is 16×46=736, and that of carbon elements is the largest.

B. Tannin is composed of carbon, hydrogen and oxygen, so the option statement is correct.

C, 1 tannin molecule consists of 76 carbon atoms, 52 hydrogen atoms and 46 oxygen atoms, and each tannin molecule contains 76+52+46= 174 atoms, so the option statement is correct.

D. According to the chemical formula of tannin, tannin is a carbon-containing compound and belongs to organic matter, so the option statement is correct.

So choose a.

Comments: This question is not difficult. Test students' ability to analyze and solve problems by combining new information and flexibly using the meaning of chemical formula and related calculations.