Concept: The word "flourished" means to thrive, prosper, or grow in a healthy and vigorous way.
Explanation:The sentence says gold trade "flourished," meaning it was thriving and successful during that period.
"Spread" means to expand to new areas, which does not match the idea of thriving.
"Started" means to begin, which is not the meaning here.
"Boomed" means to grow rapidly and achieve great success, which closely matches "flourished."
"Increased" only suggests a rise in amount, without the sense of great prosperity.
Therefore, "boomed" is the closest synonym for "flourished" in this context.
Answer: C. boomed