A biologist wants to increase the rate of his chemical reaction but has a limited amount of enzyme. He continues to increases the substrate concentration instead. Eventually, the reaction rate levels off, and he can't get it to go any faster. What prevented the rate from increasing further?

The solution ran out of enzyme
The substrate concentration reached Vmax
The products of the reaction inhibited the enzyme
The solution ran out of reactants

The substrate concentration reached Vmax

Explanation:

In this scenerio, further increase in the rate of reaction was prevented because the substrate concentration hit Vmax.

Beyond the Vmax, the substrate can no longer proceed.

The substrate is the reactant on which the enzyme works on. Enzymes are natural organic chemical  catalysts. Increasing the concentration of reactants is one of the known and proven ways to speed up the rate of reaction. Since we have limited amount of enzymes, once they all bound to the substrate, no further increase in the substrate will have an effect on the rate. At the optimum reaction point, all the available substrate will bond with the enzyme. Beyond this, the enzymes are saturated will not further any reaction.