"The author wrote his thoughts on the economic problem."
Translation:El autor escribió sus reflexiones sobre el problema económico.
Yes, because they both are translated into thoughts, but they are not synonyms. pensamiento is a direct thought (for instance I'm cold today), whereas reflexión is a complex process of thinking about a problem.
Instead of 'reflexiones', I used pensamientos and it was marked wrong. Any special reason?
Perhaps you had something else in the sentence that was incorrect, as I used pensamientos and it was fine. Maybe you wrote la problema instead of el? I did that the first time around. Or maybe you didn't conjugate escribir in the past tense?