It's d, here's why.
Stoichiometry: N2 + H2 (for instance) --> NH3
There are 2 Nitrogen atoms (or parts) of Nitrogen on the left side of the equation, and 2 Hydrogen, and only one Nitrogen but three Hydrogen on the other side. Where did the extra Nitrogen go? Where did that Hydrogen come from? The answer is Stoichiometry.
N2 + H2 --> NH3 has to be balanced, so we add coefficients to the reactants and products, which indicate in what ratio they are consumed in the reaction. They effectively multiply the subscripts on the elements.
To balance Nitrogen, we have to add a 2 to the front of NH3, so we get 2NH3. Nitrogen is balanced, but Hydrogen isn't. There are now 6 Hydrogen being produced by the reaction, so we can add a 3 to the products side, making 3H2.
Now we have N2 + 3H2 --> 2NH3, and everything is balanced.
