Explanation

The blank describes the kind of observation that would surprise people who thought Lucy was unaware, so a word that means something like sharp or observant would make sense—anything that would show that Lucy was aware. Both asinine and obtuse are nearly the opposite of what you’re looking for, so eliminate choices (A) and (C). Choices (D) and (F) have roughly synonymous meanings, but an observation that was clear wouldn’t have impressed her classmates.

Both perceptive and astute can mean observant, so choices (B) and (E) give you appropriate, equivalent sentences.