Option 1 -> This treaty was opened for signature in 1968 and entered into force in 1970, focusing on preventing the spread of nuclear weapons rather than banning them outright.
Option 2 -> The CTBT was adopted in 1996, not 2017, and focuses specifically on banning nuclear explosions for testing purposes, not the weapons themselves.
Option 3 -> The TPNW was adopted by the UN General Assembly on July 7, 2017, and is the first legally binding international agreement to comprehensively ban nuclear weapons.
Option 4 -> START is a bilateral arms control treaty between the United States and Russia (formerly Soviet Union), first signed in 1991, not a UN treaty from 2017.
Hence, Option 3: Treaty on the Prohibition of Nuclear Weapons (TPNW) -> This historic treaty was adopted in 2017 with 122 nations voting in favor, marking the first comprehensive ban on nuclear weapons under international law, and entered into force in January 2021 -> correct