What would cause entropy to decrease in a reaction?

Hint: As we know that entropy is the measure of a system's thermal energy per unit temperature that is not available for doing useful work. Since work is obtained from ordered motion of molecules, the amount of entropy is also a measure of the molecular disorder or randomness of the system.

Complete answer:
Let us discuss about entropy followed by the cause of decrement in entropy of a reaction:-
-Entropy: It is the measure of a system's thermal energy per unit temperature that is not available for doing useful work. Since work is obtained from ordered motion of molecules, the amount of entropy is also a measure of the molecular disorder or randomness of the system.
-The following are the reasons that can bring change in entropy of a reaction:-
It increases when the solid state of reactants changes to a liquid and liquid state of reactants change into gaseous form during a reaction.
It also increases when the number of moles of gaseous products is comparatively more than the reactants.
-So from the above two points we can make the conditions that would cause entropy to decrease in a reaction as follows:-
The entropy of the reaction can be decreased if the products formed are solid in nature from liquid or gaseous reactants which is most probable in case of precipitation reaction. This is because solids have the lowest entropy because the atoms are in ordered form instead of randomness as in case of gaseous state.
The entropy can also be decreased if the number of moles of gases formed as a product is less than the gaseous reactant moles which is possible in combination reaction of gases.

Note:
-Entropy is a state function which means it depends on the state of the system rather than the path being followed during a reaction and hence the above statements regarding the state of the product as well as reactant is considered in entropy changes.