Answer
The calculator displays an error message.
This is because ratio cotangent is not defined for angle $0^{\circ}$.
Work Step by Step
In degree mode, the calculator on pressing $0$ first and then $tan$, displays an error message.
Let's try finding value of $\cot 0^{\circ}$ by an alternate method-
$\cot 0^{\circ}$ = $\frac{\cos 0^{\circ}}{\sin 0^{\circ}}$
(ratio identity)
Now from table, substituting the values of $\cos 0^{\circ}$ and $\sin 0^{\circ}$-
$\cot 0^{\circ}$ = $\frac{1}{0}$
Thus $\cot 0^{\circ}$ becomes undefined because division by zero is not defined.
Calculators are also programmed accordingly.
