What did the great multitude in white robes hold in their hands before the throne?

Answer: Palm branches

A multitude too large to count, from every nation, stood before the throne in white robes, holding palm branches.

“After this I looked and saw a multitude too large to count, from every nation and tribe and people and tongue, standing before the throne and before the Lamb. They were wearing white robes and holding palm branches in their hands.”— Revelation 7:9 (BSB)