Achemical reaction in which compounds break up into simpler constituents is a reaction.
a. decomposition
b. substitution
c. double replacement
d. combination

A chemical reaction in which compounds break up into simpler constituents is D. decomposition. This should be an obvious answer - when something decomposes, it falls apart and creates smaller parts of the original thing. The same thing goes with the compounds - they will fall apart into constituents that are way simpler than them.