TLDR: A.
Understanding the idea of chemical equations is sometimes a hard concept to grasp in general chemistry because it’s typically not explained well. What a chemical equation represents is what literally happens on the subatomic level. The coefficients in front of the reactants and products mean, number for number, what you would get if you performed the reaction. It’s like an atomic recipe - you can only “make” things when you have enough ingredients. When an ingredient runs out, you can’t produce any products anymore. The ingredient that runs out the quickest is known as the “limiting reagent”, as it literally limits what you can produce.
In the equation above, 2 equivalents of “a” and 3 equivalents of “b” are consumed to produce the products. This means that “b” gets consumed faster than “a”. To make one sample of products, you need two molecules of “a” and three molecules of “b”. If you start out with four moles of each reactant (the same number of atoms), you’ll run out of “b” before “a” because the reaction consumes more molecules of “b” than molecules of “a”. Therefore, the answer would be A. You would need 6 moles of “b” to finish the reaction and use up the remaining “a” (a 2:3 ratio, just like in the balanced equation).
Hope this helps!