Final answer:
The adjective clause in the sentence 'This is the novel that I told you about' is 'that I told you about' and it modifies the noun 'novel'.
Step-by-step explanation:
The adjective clause in the sentence This is the novel that I told you about is that I told you about. It modifies the noun novel. The adjective clause provides more information about the noun by telling us which novel is being referred to.