Which words in the sentence are the complete adjective clause?

He wrote about his grandfather, who served his country in the Marine Corps.

a. who served his country in the Marine Corps

b. in the Marine Corps

c. who served his country

d. about his grandfather

The correct answer is A) who served his country in the Marine Corps.

B and D cannot be the correct answer, because they are not clauses, they are prepositional phrases.

And C cannot be correct because that is not the whole adjective clause, you have to include the whole thing. That is why A is the correct answer.