Step 1: Understand what is asked.
The assertion says that as we add more substrate, the reaction speed rises to a maximum value $V_{max}$ and then stops rising. The reason says enzyme molecules are fewer than substrate molecules. We check both and the link.
Step 2: Picture the active sites.
Each enzyme has an active site where the substrate binds. The reaction can only happen at these sites, so the number of sites sets the limit on speed.
Step 3: Add substrate slowly at first.
When substrate is low, many active sites are empty. Adding more substrate fills more sites, so the speed goes up.
Step 4: Reach saturation.
At high substrate, every active site is busy. The enzyme is now working as fast as it can. Adding even more substrate cannot help because there is no free enzyme to grab it. This top speed is $V_{max}$. So the assertion is true.
Step 5: Check the reason.
Enzymes are present in small amounts and act again and again. So enzyme molecules are indeed fewer than the many substrate molecules. This shortage of enzyme is exactly why the speed levels off. The reason is true and explains the assertion.
Step 6: Final decision.
Both are correct and the reason is the correct explanation.
\[ \boxed{\text{Both (A) and (R) are correct and (R) is the correct explanation to (A)}} \]