a chemist want to make a solution of 3.4M HCl there are tow solutions of HCl that he can find on the shelf. one has a concentration of 6.0 M while the other has a concentration of 2.0 N Which solution can the chemist use to make the desired acid

The most concentrated one, 6.0 M.

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.