If the total cost function is given by $C\left(x\right)=10x-7x^2+3x^3$ then the marginal average cost function (MAC) is given by,

Given, function is,

$C\left(x\right)=10x-7x^2+3x^3$

Here,

$\begin{array}{l}\mathrm{AC}=\frac{\mathrm{C}\left(\mathrm{x}\right)}{\mathrm{x}}\\ \mathrm{AC}=\frac{10\mathrm{x}}{\mathrm{x}}-\frac{7{\mathrm{x}}^2}{\mathrm{x}}+\frac{3{\mathrm{x}}^3}{\mathrm{x}}\end{array}$

$AC=10-7x+3x^2$

Here, marginal average cost function,

$\begin{array}{l}MAC=\frac{d}{dx}\left(AC\right)\\ MAC=\frac{d}{dx}\left(10-7x+3x^2\right)\\ MAC=-7+6x\end{array}$

Thus, the correct option is C.