The result of the Second Opium War (1856–1860) included a range of significant concessions made by China to Britain and France. Among the options you provided, the most relevant outcome is that "China legalized the opium trade."
As a result of the war and subsequent treaties, China had to accept various terms that opened up its markets to Western trade, including the legalization of the opium trade, which had been a point of contention and conflict. The other options do not accurately reflect the outcomes of the war.