Choose the correct adverb to fill in the blank:

He searched __________ for his lost keys, checking every room in the house.

frantically

The correct answer is option (2) frantically.

An adverb is needed to describe how he searched for his lost keys. "Frantically" is the appropriate adverb here as it describes the manner in which he searched, indicating that he searched with desperation or urgency. Options (1), (3), and (4) are not adverbs and do not appropriately describe the manner of searching.