A possessive noun is a word construction that shows the possession or ownership of a thing by someone. This is done by adding an apostrophe 's' ('s) to the noun to signify possession.
In the given sentence, <em>"Have you seen the mitten belonging to Margot?"</em>, the "mitten" which belongs to Margot can be changed to<em> "Margot's mitten"</em>. This shows the ownership of the mittens.
Thus, the correct sentence becomes
<em>"Have you seen </em><u><em>Margot's mitten</em></u><em>?"</em>