One mole of solute (NaCl) is dissolved in 1 litre water. The molarity of solution is
The answer $&lt;\pu{1M}$ is correct. 

Suppose you have $\pu{1L}$ ($\pu{1dm^3}$) of water and then you add your salt. When you add your salt the volume of the solution increases so now you have $\pu{1 mol}$ of $\ce{NaCl}$ in more than $\pu{1L}$ of solution. That means that you can't have a $\pu{1M}$ solution anymore.

If you want a $\pu{1M}$ solution then:


Add $\pu{1 mol}$ of salt in a flask.
Add water until you reach $\pu{1L}$ of volume.


EDIT: when you add salt, the volume of the solution increases slightly and less than what you expect(volume of water + volume of dry salt).