I believe the correct answer would be letter D.
After the fall of USSR in 1991, China and Russia engaged in reapprochement relations (they were rivals since 1961, with high possibilities of declaring war against each other, and actually engaging in a minor border war in 1969. Rumors of several invasions from both sides, although not proven, existed).
In 1994, Russian and Chinese officials signed an agreement on the Sino-Russian (Chinese-Russian) Border Management System aiming to facilitate border trade and reduce criminal activity at these regions.
In 2004, a Complementary issue of this agreement <u>stated that Russia would transfer a few island and islets to China's possession</u>, with the intention of fostering the reconciliation feeling between the two nations. The two nations hard border dispute dated since Manchuria Japanese invasion, in 1931.