There are four substances X, Y, Z and Q in a closed container, which fully react under certain conditions. The mass of each substance before and after the reaction is measured as follows: substance XYZQ. According to the data analysis in the table, the mass of Y increased by 24g-2g=22g before and after the reaction, so Y is a reactant and the mass of Y generated is 22g. The mass of C is unchanged before and after the reaction, and C may be used as the catalyst of the reaction, or it may not participate in the reaction. Similarly, it can be determined that Z is a reactant, and the mass of Z participating in the reaction is 84g-0g = 84g; Q is the product, and the mass of generated Q is14g-5g = 9g; According to the law of conservation of mass, X should be a product, and the mass of X generated is 84g-22g-9g = 51g. 。

The reactant of this reaction is Z, and the products are X, Y and Q, which accords with the characteristic of "changing from one to more" and belongs to decomposition reaction.

So choose: a.