Based on the perspective of a European living in a country conquered by the Mongols, the option that describes their view is likely:
The Mongols were responsible for the murder of millions of people in their conquest of European countries.
This option reflects the fear and resentment that could arise from experiencing the violence of the Mongol conquests firsthand. While the other options may contain elements of truth regarding trade and knowledge exchange, a European in that context would more likely focus on the devastation and loss of life caused by the invasions.