The length of side upper A upper B is 54. The measure of angle upper B

Choice C is correct. In the triangle shown, the measure of angle $B$ is $30°$ and angle $C$ is a right angle, which means that it has a measure of $90°$. Since the sum of the angles in a triangle is equal to $180°$, the measure of angle $A$ is equal to $180°-\left(30+90\right)°$, or $60°$. In a right triangle whose acute angles have measures $30°$ and $60°$, the lengths of the legs can be represented by the expressions $x$, $x\sqrt{3}$, and $2x$, where $x$ is the length of the leg opposite the angle with measure $30°$, $x\sqrt{3}$ is the length of the leg opposite the angle with measure $60°$, and $2x$ is the length of the hypotenuse. In the triangle shown, the hypotenuse has a length of $54$. It follows that $2x=54$, or $x=27$. Therefore, the length of the leg opposite angle $B$ is $27$ and the length of the leg opposite angle $A$ is $27\sqrt{3}$. The tangent of an acute angle in a right triangle is defined as the ratio of the length of the leg opposite the angle to the length of the leg adjacent to the angle. The length of the leg opposite angle $A$ is $27\sqrt{3}$ and the length of the leg adjacent to angle $A$ is $27$. Therefore, the value of $\text{tan}A$ is $\frac{27\sqrt{3}}{27}$, or $\sqrt{3}$.

Choice A is incorrect and may result from conceptual or calculation errors.

Choice B is incorrect. This is the value of $\frac{1}{\text{tan}A}$, not the value of $\text{tan}A$.

Choice D is incorrect. This is the length of the leg opposite angle $A$, not the value of $\text{tan}A$.