This is the book I was telling you . . . . . . . . . A. for B. on C. of D. about

The correct answer is: about.

The word “about” is used to introduce a topic or subject. In this case, the speaker is introducing the topic of the book they were telling the listener about.

The other options are incorrect because they do not make sense in the context of the sentence.

• The word “for” is used to indicate a purpose or destination. For example, “I bought this book for my son.”
• The word “on” is used to indicate a location or position. For example, “The book is on the table.”
• The word “of” is used to indicate possession or association. For example, “The book is of great interest to me.”

I hope this helps!