Which words in the sentence are the complete adjective clause?
the book that i liked best was the one about the big fish.
a.
that i liked best
b.
the one about the big fish
c.
the book that i liked best
d.
about the big fish

"the book that I liked best" is the complete subject clause because it replaces the subject (the proof is that it agrees with the verb was). The adjectival clause is then "the one about the big fish" because it describes the subject in more detail (which is always the function of the adjective).

Ibeileve it will be c. : )