Hey
To answer your question, this sentence violates the principles of objectivity in writing because It brings the writer into the sentence.
The sente breaks the principle of objectivity in writing because, as you might know, the definition of objectivity is something that's "not influenced by personal feelings or opinions in considering and representing facts". In writing, this translates to a summary that recapitulates the author’s points without including your own points of view, objections or criticisms.
Since the sentence here starts with an "I believe" which indicates the writer is about to state his own opinion, then this breaks he principles of objectivity in writing a summary. Therefore, the answer is D.