Final answer:
One of the signs that the Lord is fulfilling His promise to gather Israel in the last days in 3 Nephi 21: 1-11 is the coming forth of the Book of Mormon, which contains the fulness of the gospel of Jesus Christ.
Step-by-step explanation:
In 3 Nephi 21:1-11, one of the signs that the Lord is fulfilling His promise to gather Israel in the last days is mentioned. The specific sign mentioned is the coming forth of the Book of Mormon. It says that the Lord will bring forth His words to the people through the Book of Mormon and that it will contain the fulness of the gospel of Jesus Christ.
This sign is significant because it shows that the Lord is actively working to gather His people in the last days and bring them the knowledge and blessings of the gospel. The Book of Mormon serves as a key tool in this gathering process, as it provides additional witnesses of Jesus Christ and His teachings.
Furthermore, the gathering of Israel in the last days involves bringing together not only the descendants of the ancient Israelites but also people from all nations who are willing to accept the gospel of Jesus Christ. So, the sign of the Book of Mormon serves as a fulfillment of the Lord's promise to gather Israel in the last days.