Final answer:
Of the options given, the decrease in enzyme concentration would decrease the Vmax of a reaction. Increasing the substrate concentration initially increases the rate of the reaction, but there is a saturation point where further increase in substrate concentration does not increase the rate further. Temperature and pH can also affect Vmax.
Step-by-step explanation:
Of the options given, the decrease in enzyme concentration would decrease the Vmax of a reaction.
Increasing the substrate concentration, on the other hand, would initially increase the rate of the reaction. However, once all enzyme molecules are saturated with substrate, increasing the substrate concentration further will not increase the rate any further.
Increasing the temperature would generally increase the rate of an enzyme-catalyzed reaction, up to a point. At high temperatures, the enzyme can become denatured and lose its activity, leading to a decrease in Vmax. Lastly, decreasing the pH could also decrease Vmax as enzymes often have an optimum pH range, and deviation from this range can negatively affect their activity.