An elder tells the weeping John that who has triumphed to open the scroll?

Answer: The Lion of the tribe of Judah

"Do not weep! Behold, the Lion of the tribe of Judah, the Root of David, has triumphed to open the scroll and its seven seals."

“And I began to weep bitterly, because no one was found worthy to open the scroll or look inside it. Then one of the elders said to me, “Do not weep! Behold, the Lion of the tribe of Judah, the Root of David, has triumphed to open the scroll and its seven seals.””— Revelation 5:4-5 (BSB)