Answer
$\tan$ is to be filled in the blank.
Work Step by Step
$$A72^\circ=\cot18^\circ\hspace{1cm}(1)$$
We notice that $$72^\circ+18^\circ=90^\circ$$
That means we can rewrite $(1)$ as
$$A72^\circ=\cot(90-72^\circ)\hspace{1cm}(1)$$
Now from the Cofunction Identity:
$$\tan\theta=\cot(90^\circ-\theta)$$
So $$A=\tan$$
In other words, we fill in the blank with $\tan$.
