The most accurate statement that summarizes the figures and events that led to the fall of the Roman Republic is:
The Roman Republic fell due to the first civil war and political struggles between the Populares and Optimates.
This statement captures the significant civil conflict (the first civil war between Caesar and Pompey) as well as the ongoing political struggles between the two factions, the Populares and the Optimates, which were central to the political instability leading to the Republic's downfall.