Given below are two statements. One is labelled Assertion (A) and the other is labelled Reason (R).
Assertion (A): The velocity of an enzymatic reaction increases with the increase in substrate concentration until it reaches a maximum velocity (V_max).
Reason (R): At V_max, all enzyme molecules are saturated with substrate molecules, and no free enzyme molecules are available to bind with additional substrate molecules. 
In light of the above statements, choose the correct answer from the options given below.

Assertion (A) is correct and describes the initial linear relationship between substrate concentration and reaction velocity observed in enzyme kinetics. As the substrate concentration increases, the rate of the enzymatic reaction also increases because more substrate molecules are available to bind with enzyme molecules and form enzyme-substrate complexes. However, at a certain point, the reaction velocity reaches a maximum value (V_max), after which it remains constant regardless of further increases in substrate concentration.

Reason (R) is also correct. At V_max, the enzyme active sites are fully occupied by substrate molecules, resulting in the maximum possible rate of formation of enzyme-substrate complexes. Because all enzyme molecules are engaged in catalyzing the reaction, there are no free enzyme molecules available to bind with additional substrate molecules, and the reaction velocity remains constant.

Therefore, Reason (R) provides a valid explanation for why Assertion (A) is true. The saturation of enzyme molecules with substrate molecules at V_max leads to the observed behavior in enzyme kinetics where the reaction velocity reaches a maximum value.