Choose the correct adverb to fill in the blank:

She arrived __________ to catch the early morning train.

early

The correct answer is option (1) early.

An adverb is needed to describe how she arrived to catch the early morning train. "Early" is the appropriate adverb here as it describes the manner in which she arrived, indicating that she arrived before the expected time. Options (2), (3), and (4) are not adverbs and do not appropriately describe the manner of arriving.