The best option is D studied.
This word denotes a measured, even-handed approach to the issue. So, it should be :Current immigration policy must be studied before new proposals can be made.
The correct answer is A. Nonfiction writers are more often free to move about in time and space.
B is not correct because that is usually the characteristic of nonfiction, rather than fiction. C is incorrect because usually writers of fiction can embellish the facts because they are not based on actual information. D is incorrect because sometimes writers of nonfiction can express their passionate opinions, especially in essays.
Idiom is an expression particular to a region
