Areaction that is essentially the opposite of a synthesis reaction describes which of the following? a) oxidation-reduction b) decomposition c) displacement d) combustion

Answer is: b) decomposition.

Synthesis is reaction of two or more substances combining to make a more complex substance.

Decomposition is reaction where one substance is broken down into two or more simpler substances.

For example, balanced chemical reactio:

CaCO₃(s) → CaO(s) + CO₂(g).

Chemical decomposition is the separation of a single chemical compound (in this example calcium carbonate) into its two or more simpler compounds (in this example calcium oxide and carbon dioxide).

Combustion is reaction with oxygen.

Single Displacement is reaction where  neutral element metal or nonmetal become an ion as it replaces another ion in a compound.  

Double displacement reactions (more reactive metals displace metals with lower reactivity).

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.