Final answer:
The crop that was so controversial that it led to attempts by European and Chinese monarchs to ban its use was tobacco. These attempts were largely due to health concerns and moral considerations.
Step-by-step explanation:
The question asks about a crop that was considered so controversial that it led to attempts by monarchs in Europe and China to ban its use. The correct answer is a. Tobacco. Tobacco became incredibly popular in Europe after its introduction from the Americas, but it also garnered a negative reputation due to health concerns and moral considerations. This led to European monarchs, such as King James I of England, to speak out against its use and attempt to control its consumption. Similarly, in China, the Ming and Qing dynasties made repeated efforts to prohibit the use of tobacco, although these measures were often hard to enforce due to its widespread popularity.