to reverse the balance of trade with china, british merchants sold
to chinese smugglers.
tea
opium
silk
silver

The correct answer is Opium.

Explanation:

 Britain began selling opium to smugglers during the 18th and 19th centuries, as the Chinese government had banned it because of the great addiction it caused.

The money they obtained from this sale was used to buy Chinese products that could easily be sold in the West.

This opium was sold to smugglers who in return offered them gold that Britain used to buy Chinese products, such as porcelain, tea, etc. to do business in Europe.

Year after year the amount of opium sold to China was growing extraordinarily, to the point where an opium traffic network was generated.

By 1917 this trade ended almost definitively, due to an agreement that China signed with India (where the opium was grown) where they prohibited continuing to do this practice, and thus curb traffic.

islam spread slowly through much of africa through trade and preaching.