IMO it's D.
If most mail arrives within 3+ days, and most correctly addressed mail arrives within 2 days, then most mail is cannot be correctly addressed. This means, D - a large proportion of mail is incorrectly addressed.
B tries to claim that a lot of the correctly addressed mail is damaged. However, this is not possible as we know most correctly addressed mail arrives in 2 days, which means it is not damaged.
As for C, it plays on the first line in the argument, but reverses it. Notice that the fact most A is B does not automatically mean most B is A. So the fact most correctly addressed mail arrive within 2 days, doesn't mean most mail arriving within 2 days is correctly addressed. There could be a large amount of 'lucky' incorrectly-addressed mail which also arrives within two days. This does not contradict the fact most (but not all) mail takes longer than 3 days - for example:
10% correctly-addressed mail arriving within 2 days
14% 'lucky' incorrectly-addressed mail arriving within 2 days
1% damaged correctly-addressed mail arriving within 2+ days
75% incorrectly-addressed mail arriving withing 3+ days
In this example, most mail arriving in 2 days is incorrectly-addressed, eliminating C.
