Answer
We fill in the blank with $\cos$.
Work Step by Step
$$A33^\circ=\sin57^\circ\hspace{1cm}(1)$$
We notice that $$33^\circ+57^\circ=90^\circ$$
That means we can rewrite $(1)$ as
$$A33^\circ=\sin(90-33^\circ)\hspace{1cm}(1)$$
Now from the Cofunction Identity:
$$\cos\theta=\sin(90^\circ-\theta)$$
So $$A=\cos$$
In other words, we fill in the blank with $\cos$.
