$\mathrm{Cu}$ reacts with ${\mathrm{HNO}}_3$ according to the equation $\mathrm{Cu}+{\mathrm{HNO}}_3⟶\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2+\mathrm{NO}+{\mathrm{NO}}_2+{\mathrm{H}}_2\mathrm{O}$ If $\mathrm{NO}$ and ${\mathrm{NO}}_2$ are formed in a $2:3$ ratio, what is the coefficient for $\mathrm{Cu}$ when the equation is balanced with the simplest whole numbers?

$\mathrm{Cu}$ reacts with ${\mathrm{HNO}}_3$ according to the equation 

$\mathrm{Cu}+{\mathrm{HNO}}_3\to \mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2+\mathrm{NO}+{\mathrm{NO}}_2+{\mathrm{H}}_2\mathrm{O}$

If $\mathrm{NO}$ and ${\mathrm{NO}}_2$ are formed in a 2:3 ratio, then 9 is the coefficient for Cu when the equation is balanced with the simplest whole numbers.

Total $R\to 90\to 2$

Hence, the correct option is D.