Final answer:
A proper adjective that can be used to complete the sentence is 'Japanese'. Seaweed is commonly used in Japanese sushi, which is a traditional dish in Japan.
Step-by-step explanation:
A proper adjective that can be used to complete the sentence is Japanese.
Seaweed is commonly used in Japanese sushi, which is a traditional dish in Japan. It is important to use a proper adjective in this context as it describes an attribute of the noun 'sushi' that originates from Japan.