Explanation:

• Special trading arrangements were needed for Northern Ireland after the UK voted for Brexit in 2016. This is because it's the only part of the UK with a land border with an EU country - the Republic of Ireland. Before Brexit, it was easy to transport goods across this border because both sides had the same EU trade rules. 
• No checks or paperwork were necessary. After Brexit, a new system was needed because the EU has strict food rules and requires border checks when certain goods -arrive from non-EU countries. Hence, the UK and the EU agreed to sign the Northern Ireland Protocol in 2019 as part of the Brexit withdrawal agreement, which is now part of international law.

Hence option 3rd is correct.