X pro is the flagship series with best coating and reflectivity. Nano D series is better for photography beginners. 

Yes, choose size 67mm. 

1-9 Stops. 

From 1-5 stops, there will be a slight X cross for lens under 24mm. An X cross will appear at the highest stop and can be removed by rotating the filter ring, which is a normal situation instead of a quality issue. 

First, K&F Concept ND2-ND400 lens filter allows changing shutter speed based on the effect you want with the best sensibility and the same aperture. Second, K&F Concept ND2-ND400 lens filter allows creating a blurred background with a large aperture while the shutter speed remains the same. Third, K&F Concept ND2-ND400 lens filter is designed to prevent overexposure and maintain image purity and quality at the same time. *Like a zoom lens, the variable nd filters are suitable for multiple shooting situations. If you're looking for perfect image quality, a fixed ND filter is recommended.