Choose the correct adverb to fill in the blank:

She completed the assignment __________, ensuring accuracy and attention to detail.

thoroughly

The correct answer is option (2) thoroughly.

An adverb is needed to describe how she completed the assignment. "Thoroughly" is the appropriate adverb here as it describes the manner in which she completed, indicating that she completed with completeness or carefulness. Options (1), (3), and (4) are not adverbs and do not appropriately describe the manner of completion.