There are four chapters that are extraneous to the structure of the book.

Answer: Option B

Extraneous : irrelevant or unrelated to the subject being dealt with.
Relevant : closely connected or appropriate to what is being done or considered.
Integral : necessary to make a whole complete; essential or fundamental.
Important : of great significance or value.
Needful : necessary; requisite.

Antonym of Extraneous is Integral