Final answer:
To study the effect of substrate concentration on reaction rates, the variable that is varied is D) Substrate amount. This is done by keeping enzyme concentration constant and altering the amount of substrate, observing how the rate of reaction increases with substrate concentration until the enzyme becomes saturated.
Step-by-step explanation:
When studying the effect of substrate concentration on reaction rates, substrate amount is the variable that is changed to measure this effect. This is evidenced by setting up a series of reaction tubes each with the same concentration of enzyme but with different substrate concentrations. As the substrate concentration increases, the reaction rate also increases until the point where the enzyme becomes saturated with substrate. At this point, even with further increases in substrate amount, the reaction rate will not increase and becomes constant. This relationship is typically depicted in a hyperbolic curve when reaction velocity is plotted against substrate concentration.
It's important to note that when we alter the enzyme concentration, we also affect the reaction rate, but this is not the focus when studying the effect of substrate concentration. Enzyme concentration typically shows a linear relationship with reaction rate, assuming there is an excess amount of substrate present and other factors are kept constant.