The correct option is B.
Single replacement reaction is a type of chemical reaction, in which one element is substituted for another element in a compound. There are usually two reactants, one is a pure element and the other one is an aqueous compound. The products formed is usually made up of another pure element and a new compound. In the option B, you will notice that, in the reactant part, Mg is a pure element while in the product side, H2 is a pure element.
