The TP curve increases initially, at an increasing rate (till point M) and then at a decreasing rate, reaches a maximum at T and then falls.
When the TP increases at an increasing rate, the marginal product increases. However, when TP increases at a decreasing rate, MP declines. When TP is maximum, MP is zero and becomes negative when TP begins to decline. This is shown in the diagram.
![](https://www.sarthaks.com/?qa=blob&qa_blobid=4242085878331471341)