Final answer:
Synthesis reaction' as the equation A + X → AX represents two substances combining to form one new substance. The correct answer is 'c.
Step-by-step explanation:
The equation A + X → AX is the general equation for a synthesis reaction. This type of reaction involves two or more substances combining to form a single new substance.
In contrast, a decomposition reaction, represented by the equation AB → A + B, involves a single compound breaking down into two or more new substances. In single-replacement reactions, an element replaces a similar element in a compound, following a general form of A+BC → AC + B or Y+XZ → XY+Z when the replacing element is a nonmetal. Therefore, the correct answer to the question is 'c. Synthesis reaction'.