The sentence that is written in a formal style and objective tone is:
"Book designers avoid line breaks that would hinder a reader’s comprehension of the text when they lay out a manuscript."
Reason: This sentence uses formal vocabulary ("hinder", "comprehension") and maintains an objective tone by avoiding personal pronouns and subjective language. It presents information in a clear and professional manner, focusing on the role of book designers without personal opinions or informal expressions. The other sentences contain more informal phrasing or personal references that detract from a formal style.