The operator should stand at a distance of _______ while
taking X-ray.
Means for Reducing X-Ray Exposure: 

Use selection criteria to assist in determining type and frequency of radiographic examinations. 
Use E/F-speed film or digital sensors. 
Use holders to support film or digital sensors intraorally.
Make exposures with 60 to 70 kVp. 
Replace short pointed aiming tubes with open-ended aiming cylinders. 
Use rectangular collimation for periapical and bitewing images. 
Use thyroid collars. 
Stand at least 6 feet (2 m) away from patient and away from the X-ray machine (preferably out of X-ray room) when making exposure. 
With film, use time-temperature film processing rather than "sight" processing, or use an automatic processor. 
Use rare-earth screens for panoramic and cephalometric film imaging or use digital systems. 
Reduce cone-beam CT beam field of view to region of interest. 

Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 33
6 feet
8 feet
10 feet
2 meters