Omar writes science fiction novels. He notices he has been paying for clicks when customers search for "science textbooks." Which keyword match type should Omar use to ensure ads are not served to customers searching for science textbooks?

Negative phrase match is the correct choice because it prevents ads from showing for searches containing the specified phrase, even if other words are present. In this case, adding "science textbooks" as a negative phrase match keyword would stop Omar's ads from appearing when users search for that exact phrase or variations like "buy science textbooks online," but still allow ads for relevant searches like "science fiction novels." Broad match, phrase match, and exact match are all positive keyword match types that would include "science textbooks" in their targeting, either broadly, as a phrase, or exactly, which is the opposite of Omar's goal.