According to the chart below, what were the TWO most common reasons for changes of leadership during the third century of Rome?

The most common reasons were assassination or death in a battle.

Explanation:

The crisis in the Roman Empire started after the death of Marcus Aurelius.

Corrupted system led to creation of unstable country.

That is why until reign of Diocletian many leaders were changed and as we can see from the chart due to many conflicts and quarrels.

That is why country suffered a lot and never fully recovered.

b: the exchange of coined money