The GRE deliberately plants a pair of words that are synonyms of each other but do not fit the sentence. It is bait for students who look for "two words that match" without first checking that they match the sentence.
Once dismissed as ______ by critics who prized only the grand and the monumental, her miniature sculptures are now studied as masterpieces of compression.
The sentence's logic (contrast: "Once dismissed … now masterpieces") tells you critics once judged the work negatively — as trivial or slight. Predict: "insignificant."
Now imagine the six choices include grandiose and monumental. Those two are synonyms of each other — and the sentence even contains the words "grand" and "monumental," which makes them feel relevant. But they mean the opposite of what a dismissive critic would say. They are the trap pair. The correct pair would be words like negligible and paltry (both meaning trivially small).
Occasionally three choices seem plausible. Two of them will be near-synonyms of each other; the third will be a near-synonym of nothing on the list, or will shade the meaning slightly differently. Choose the two that are closest to each other and to your prediction. If two candidate pairs both seem to work, you have under-specified your prediction — reread the signal words and sharpen it.