A. According to the fact that two n-propyl molecules J generated after the reaction contain two right N atoms and two right H atoms, and one right B molecule J of the reactant only contains two right N atoms, it can be judged that three right A molecules j*** participating in the reaction contain two right H atoms, so each right A molecule consists of two right H atoms, and the chemical formula of substance A is H2; Therefore, A is incorrect;

Eight, from the perfect microscopic diagram S of the reaction, we can know that the species and quantity of atoms are unchanged before and after the reaction, so the reaction conforms to the law of conservation of mass; Therefore, eight is incorrect;

C. The molecules before the change reaction are hydrogen molecules and nitrogen molecules, while the molecules after the reaction are ammonia molecules. The molecules change before and after the reaction, so the chemical change J molecule can be subdivided into particles that are not the smallest; Therefore, c is incorrect;

D, according to the molecular structure S, the chemical formula of the substance A is H2, the chemical formula of the substance B is N2, and the chemical formula of the generated two substances is NH3, so the chemical equation of the reaction can be written as 3H2+N2═2NH3, and the reaction is a combination reaction of two substances to generate one substance; Therefore, d is correct;

So choose D.