It is difficult to authenticate these old manuscripts.

Answer: Option D

Illustrate : provide (a book, newspaper, etc.) with pictures.
Edit : prepare (written material) for publication by correcting, condensing, or otherwise modifying it.
Elucidate : make (something) clear; explain.
Verify : make sure or demonstrate that (something) is true, accurate, or justified.
Authenticate : prove or show (something) to be true, genuine, or valid.

Synonym of Authenticate is Verify