For the reaction:

$\mathrm{A}+2\mathrm{B}\to \mathrm{C}$

5 moles of A and 8 moles of B will produce

Given reaction is a balanced chemical reaction and states that one mole of A reacts with 2 moles of B. Moles of B required to react with 5 moles of A is 10 moles but provided moles is 8 moles. Therefore, B is the limiting reagent and will decided the concentration of C.

According the given reaction, 2 moles of B produce one mole of C. Moles of C produced by 8 moles of B is:

$\begin{array}{ccc}\mathrm{Mol} \mathrm{of} \mathrm{C}& =& 8 \mathrm{mol} \mathrm{of} \mathrm{B}\times \frac{1 \mathrm{mol} \mathrm{of} \mathrm{C}}{2 \mathrm{mol} \mathrm{of} \mathrm{B}}\\ & =& 4 \mathrm{mol} \mathrm{of} \mathrm{C}\end{array}$