Answer:
d. a series of plebeian revolts.
Step-by-step explanation:
From the late second century B.C. onward, Rome deal with a lot of internal turmoil, all while extending its power.
This period is called Crisis of the Roman Republic.
Most of the causes were related to plebian revolts, against slavery, wars, land reform, ...
So the correct answer is:
d. a series of plebeian revolts.