The answer is Naturalist fiction.
I think the answer is "In 1867, the United States bought Alaska from Russia for $7 million". When you ask the question "when" the question is asking about time. In this excerpt, when is answered by "in 1987". while the sentence supported it is the event that happened during that time, which is " The US bought Alaska from Russia for $7 million.
Text me it I am pretty sure I can help you out