Which  best  describes a chain reaction associated with a nuclear reaction?

I think fission chain reaction is the correct answer.

1. two major accomplishments of the roman republic:

after the conflict of the orders, plebeians earned significant civil rights and the power of the patricians decreased. during this period, roman control extended from the city limits to the whole of the mediterranean world.

two major accomplishments of the roman empire:

pax romana (27 bc to 180 ad), which was a long period of relative peace and stability. the empire achieved its greatest territorial extent and its population reached a maximum of up to 70 million people (a third of the world’s population).

2. two failures of the roman republic:

constant state of quasi-war against etruscans, latins, gauls and carthaginians. several internal political struggles, including bloody civil wars.

two failures of the roman empire:

constant change of emperors. 193 ad is known as the year of the five emperors, and during the crisis of the third century, 26 men were declared emperors over a fifty-year time span. during this period, the empire was divided in two: western and eastern roman empires.

3. how they could have improved:

republic: if political change had happened faster and more easily, maybe civil wars and revolts would not have been as necessary. empire: clearer succession laws could have prevented the constant change of emperors.