In Revelation 7:14, those in white robes had washed their robes and made them white in what?

Answer: The blood of the Lamb

An elder said they had come out of the great tribulation and washed their robes and made them white in the blood of the Lamb.

“Then one of the elders addressed me: “These in white robes,” he asked, “who are they, and where have they come from?” “Sir,” I answered, “you know.” So he replied, “These are the ones who have come out of the great tribulation; they have washed their robes and made them white in the blood of the Lamb.”— Revelation 7:13-14 (BSB)