$3$ gm of Mg is burnt in a closed vessel containing $3$ gm of oxygen. The weight of excess reactant left is :

The correct option is A $1.0$ gm of oxygen
$\underset{24g}{Mg}+\underset{16g}{\frac{1}{2}{O}_2}⟶MgO$

$1gm$ $Mg$ requires $\frac{16}{24}g$ oxygen

So, $3gm$ $Mg$ requires $3\times \frac{2}{3}=2gm$ of oxygen

So, $3gm$ $Mg$ and $2gm$ of oxygen reacted and $1gm$ of oxygen left.

Hence the correct option is B.