Final answer:
The category of nontariff barriers that primarily checks quality issues is Technical barriers to trade (TBT). Technical barriers to trade refer to regulations and standards imposed by a country to protect the health, safety, and environment of its citizens.
Step-by-step explanation:
The category of nontariff barriers that primarily checks quality issues is Technical barriers to trade (TBT). Technical barriers to trade refer to regulations and standards imposed by a country to protect the health, safety, and environment of its citizens. These barriers can include requirements related to product quality, labeling, packaging, and testing. In the case of the rejected plywood shipment, the Japanese customs are enforcing technical barriers to trade by rejecting the plywood due to the large knotholes, which do not meet their quality standards.