The optical density of gross fog is:
An unexposed film, when processed, shows some density. This appearance is caused by the inherent density of the base and added tint and the development of a few unexposed silver halide crystals. This minimal density is called base plus fog and typically is 0.2 to 0.3. 
 
White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition
0.6-3.0
0.2-0.3
0.2-0.6
None of the above