What caused relations with China to diminish during this period?

The correct choice highlights the geopolitical tensions arising from China’s territorial claims in the South China Sea, which have increasingly threatened U.S. naval interests in the region. During this period, China has asserted its claims over significant portions of the South China Sea, leading to confrontations with neighboring nations and concerns from the United States regarding freedom of navigation and the strategic positioning of military assets. The U.S. views this as a direct challenge to its influence in Asia and its commitment to allies in the region, making it a central reason for the deterioration of relations.

The other options, while reflecting various aspects of U.S.-China relations, do not encompass the primary issues leading to the decline in diplomatic rapport. Disagreements over environmental policies have not been as significant as territorial disputes. China's economic reforms, which have mainly integrated it into the global economy, have not generally been a source of conflict. U.S. support for Taiwan's independence is also a relevant issue, but the more immediate and pressing concern recognized during this period has been the maritime disputes in the South China Sea.