To find the sum of two angles in terms of tangent, we can use the tangent addition formula:

$$ \tan (A + B) = \frac{\tan A + \tan B}{1 - \tan A \cdot \tan B} $$

Let's compute $\tan (A + B)$ using the given $\tan A$ and $\tan B$:

$$ \tan A = \frac{1}{\sqrt{x(x^2+x+1)}} $$

$$ \tan B = \frac{\sqrt{x}}{\sqrt{x^2+x+1}} $$

Now, we can apply the addition formula:

$$ \tan (A + B) = \frac{\frac{1}{\sqrt{x(x^2+x+1)}} + \frac{\sqrt{x}}{\sqrt{x^2+x+1}}}{1 - \frac{1}{\sqrt{x(x^2+x+1)}} \cdot \frac{\sqrt{x}}{\sqrt{x^2+x+1}}} $$

$$ \tan (A + B) = \frac{\frac{1 + \sqrt{x^2}}{\sqrt{x(x^2+x+1)}}}{1 - \frac{1}{(x^2+x+1)}} $$

$$ \tan (A + B) = \frac{\frac{\sqrt{x^2} + 1}{\sqrt{x(x^2+x+1)}}}{\frac{(x^2+x+1) - 1}{(x^2+x+1)}} $$

$$ \tan (A + B) = \frac{(x + 1)\sqrt{(x^2+x+1)}}{(x^2 + x){\sqrt{x}}} $$

$$ \tan (A + B) = \frac{{(x + 1)}\sqrt{(x^2+x+1)}}{x{(x + 1)}{\sqrt{x}}} $$

$$ \tan (A + B) = \frac{\sqrt{x^2+x+1}}{x^{3/2}} $$

$$\tan \mathrm{C} = \sqrt{x^{-3}+x^{-2}+x^{-1}} = \sqrt{\frac{1}{x^3}+\frac{1}{x^2}+\frac{1}{x}} = \sqrt{\frac{1+x+x^2}{x^3}} = \frac{\sqrt{x^2+x+1}}{x^{3/2}}.$$

We see that:

$$ \tan (A + B) = \tan C $$

Since tangent is positive and all the angles $A$, $B$, and $C$ are in the first quadrant, we can say that:

$$ A + B = C $$

Hence, the answer is:

Option A : $C$