A "chloride" by definition is some compound with chlorine in it. So therefore, your mystery element and chlorine form a compound
We also know that the molar mass of any compound is the sum of the molecular weights of its constituent elements. Therefore, we have:
The molar mass of
If you look on a periodic table, you'll see that that molar mass is closest to that of carbon. So carbon is your mystery element.
What if you had to take into account the possibility of multiple
Hope that helps :)