If an enzyme in solution is saturated with substrate, the most effective way to obtain a faster yield of products is to
a. add more of the enzyme.
b. heat the solution to 90°c.
c. add more substrate.
d. add a noncompetitive inhibitor.

a. add more of the enzyme.

Explanation:

Enzymes speed up the rate of reaction by lowering the activation energy for the reaction. They are not used themselves in the reaction. They are specific to substrate molecule. Substrate molecule binds to enzyme's active site and they undergo the reaction to form the product and release back the enzyme.

Rate of reaction depends on both substrate and enzyme concentration. Maximum rate of reaction is reached when all the active sites of enzyme molecules have been occupied by the substrate molecules which means that they are saturated. If more substrate is added at this point it wont have any effect on rate of reaction since there are no free active sites. Hence more enzyme is required to be added so that extra substrate can be utilized and rate of reaction can further be increased.  

oxygen +1 most likely