A ______reaction occurs when the reactants are reproduced by reaction of products 1.exchange 2.decomposition 3.synthesis 4.reversible

Your answer is #4, Reversible.  A reversible reaction is one in which the reactants form products, and the products can be transformed back into reactants. In other words, it is a chemical reaction that can be made to proceed in either direction contingent upon the correct conditions. So if reactants A and B create products C and D,

A + B --> C + D

and reactants C and D can create products A and B,

A + B <-- C + D

then the equation can be denoted with a double arrow, showing it goes in both directions:

A + B <--> C + D