The correct response to fill in the blank is "éramos".
So the complete sentence would be: "Ustedes éramos muy traviesas!"
However, note that "ustedes" (you all) is used with the verb form of "ustedes" which should be "eran," so the best option based on the given choices should actually be:
"Ustedes eran muy traviesas!"
But from the options you provided, "éramos" is also highlighted suggesting the context might be about your mother reflecting on when she was a child with you two.
So, if you have no choice but to stick with your options, "éramos" is the closest although contextually, "eran" is more correct for "ustedes".
Let me know if I can assist you with anything else!