The correct option is B Compound Sentence
A compound sentence is made up of independent clauses, i.e., both
clauses can stand on their own with complete meaning. They use a coordinating
conjunction to combine the clauses. The given sentence contains independent clauses "Her mother thought the book had a very good suspense element" and "(her mother) suggested her friends to read it." (when the subject is common, it is not repeated again). Hence it is a compound sentence and B is correct. A simple sentence contains only one clause and a complex sentence contains both independent and dependent clauses. Hence options A and C are incorrect. Option D is incorrect because the correct answer lies in B.