A. 1/2 inchB. 5/8 inchC. 7/8 inchD. 1 inch
The answer is: for the best and assured best connection, the end of the copper pipe should be inserted to the maximum depth of the fitting, and then swaged or tightened. You don't say if this is a 2-piece ferrule or a single ferrule, so providing a measure of depth is impossible. I have never seen a small diameter compression fitting that is deep enough to go more than about 12mm or 1/2 inch. You might be OK going slightly less than a 1/2 inch, but never less than 3/8 inch.
