The historian's latest monograph, while meticulously researched and rich in archival detail, proved somewhat ______ due to its overly academic prose and lack of engaging narrative.

✓ Correct Answer: Option B

The sentence describes a monograph that is difficult to understand ('overly academic prose,' 'lack of engaging narrative'). 'Esoteric' means intended for or likely to be understood by only a small number of people with specialized knowledge, fitting this context.