Final answer:
The quality issues that led to the rejection of the plywood shipment by Japanese customs fall under the category of Technical Barriers to Trade, which is a type of nontariff barrier.
Step-by-step explanation:
The shipment of plywood has been rejected by Japanese customs due to quality issues related to the size of knotholes. The category of nontariff barriers that would primarily check such quality concerns is C. Technical Barriers to Trade. These barriers include rules, regulations, and inspections that countries implement to ensure imported products meet specific standards. Thus, technical barriers to trade can limit imports just as effectively as high tariffs or quotas, ensuring that products conform to the importing country's safety, health, or technical standards.