This book is said _____ in the fourteenth century ?

The sentence 'This book is said _____ in the fourteenth century' requires a completion that conveys how the book is perceived regarding its authorship and timing. Among the options provided, "to have been written" (B) is the only grammatically and contextually appropriate choice.

First, let's break down option B: "to have been written." This phrase effectively utilizes the perfect infinitive form, which indicates an action that was completed in the past relative to another past event. In this context, it suggests that the book's authorship happened before the current acknowledgment or discussion of it, thus aligning perfectly with the temporal reference to the fourteenth century.

In contrast, options A ("many times") and C ("when it was written") do not fit syntactically into the structure of the sentence. Option D ("by being written") introduces unnecessary complexity and does not accurately convey the notion of judgment or belief about the authorship of the book in a historical context. Therefore, option B is the best way to express the idea that people assert the book existed from an earlier time, specifically in the fourteenth century.