Which graph most likely shows an ecosystem that has underwent a primary succession event during the observation period?

Hello! The answer is D

Primary succession is an event that occurs when an ecosystem is devoid of all organisms and a slow growth of the ecosystem is observed until it reaches a climax population. The most common example of a primary succession is the land development after a volcano eruption. 

In graph D a slow increase in the number of individuals is observed until the graph reaches a maximum, that is called the climax population. This is why this graph shows an ecosystem that has undergone a primary succession during the observation period.