The correct answer is option 1: "At Night All Blood is Black." This book was declared the winner of the 2021 International Booker Prize.

Option 2, "The Dangers of Smoking in Bed," is not the correct answer. While it may be a book, it did not win the 2021 International Booker Prize.

Option 3, "When We Cease to Understand the World," is also not the correct answer. Although it may be a book, it did not win the 2021 International Booker Prize.

Option 4, "The War of the Poor," is not the correct answer either. It may be a book, but it was not declared the winner of the 2021 International Booker Prize.

Therefore, the correct answer is option 1, "At Night All Blood is Black." This book was chosen as the winner of the 2021 International Booker Prize.