Which search query would trigger an ad based on this keyword:[London portrait photographer]?

The correct answer is "London portrait photographer" because the keyword is set as a broad match modifier (BMM) or phrase match, indicated by the brackets. This means the search query must contain all the words in the keyword, in any order for BMM, or in the exact order for phrase match (though Google often treats BMM and phrase match similarly now). "London portrait photographer" is an exact match for the keyword. "Portrait photographer London" would also likely trigger the ad if it's BMM, but the question asks which query would trigger it, and the exact match is the most direct. "London photographer" is too broad and misses the crucial word "portrait," so it would not trigger an ad for this specific keyword.