Answer: True
Step-by-step explanation:
Looking at the Production Function you will notice that Output increases at different rates to input added. First output increases at an increasing rate then at a constant rate then at a decreasing rate.
This shows the concept of the Law of Diminishing Marginal returns because as more input(labor) was added, at some point the output started increasing at a decreasing rate till it gets to a point where marginal returns will be negative.
The reason for this is that each worker is now working with less capital than before (assuming capital doe not change) and so will only produce less.