One mole of solute (NaCl) is dissolved in 1 litre water. The molarity of solution is One mole of solute ($\ce{NaCl}$) is dissolved in $\pu{1 L}$ of water. The molarity of the solution is:
A) $\pu{&lt;1M}$
B) $\pu{&gt;1M}$
C) $\pu{=1M}$
D) $\pu{=2M}$

My answer: It’s a fairly simple question so, I did what anybody would do.
$\mathrm{Molarity}=\frac{\text{No. of moles}}{\text{volume of solution}}$
$\mathrm{Molarity}=1$
That is the obvious answer, and it’s probably right, but the answer given is $\pu{&lt;1M}$. I just wanted to confirm that the answer given is right or wrong. If it is right, why?
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).