Answer:
The most concentrated one, 6.0 M.
Step-by-step explanation:
A simple and reliable way to produce a solution of HCl (or anything else, for that matter) is to use a more concentrated solution and dilute it.
In this case the chemist could take a portion of the 6.0 M solution and dilute it by adding water, until the concentration is 3.4 M.
Such a process would not be possible with the 2.0 N (which is the same as 2.0 M for HCl) solution.