Choose the correct adverb to fill in the blank:

The teacher explained the concept __________ to ensure that all the students understood.

clearly

The correct answer is option (2) clearly.

An adverb is needed to describe how the teacher explained the concept. "Clearly" is the appropriate adverb here as it describes the manner in which she explained, indicating that she explained in a way that was easy to understand. Options (1), (3), and (4) are not adverbs and do not appropriately describe the manner of explaining.