Answer:
B) this book had a lot of useful information.
Step-by-step explanation:
A) is informal because "tons" used in this context to mean 'a large amount' is an informal word
C) is informal because 'book is' is contracted to become "book's". Contractions are not supposed to be used in formal language.
D) is informal because formal language does not use first person pronouns (ie. Don't use I, we, us, etc) or refer to oneself, however this sentence uses "me" which refers to the writer.
Therefore, the answer must be B)