The correct answer is option 1: London. London is recognized as the first urban settlement in the world to reach a population size of one million or more, making it the first "million-plus city" in history.

Option 2, Paris, is not the correct answer. While Paris is also a major city with a rich history, it did not reach a population size of one million before London did.

Option 3, New York, is also not the correct answer. While New York is one of the largest cities in the world today, it did not reach a population size of one million before London did.

Option 4, Washington D.C., is not the correct answer either. Washington D.C. is the capital of the United States and is not considered to be one of the world`s largest or most populous cities.

In summary, the first urban settlement in the world to achieve a population size of one million or more was London, leading it to be recognized as the first "million-plus city" in history.