Answer
$\cos75^\circ$ is the cofunction needed to find.
Work Step by Step
$$\sin15^\circ$$
Cosine is the cofunction of sine. That means the question asks to write $\sin15^\circ$ in terms of sine and an angle. In other words, what is the value of $\theta$ with which
$$\cos\theta=\sin15^\circ\hspace{1cm}(1)$$
According to Cofunction Identity: $\cos\theta=\sin(90^\circ-\theta)$
Apply this to the equation $(1)$:
$$\sin(90^\circ-\theta)=\sin15^\circ$$
$$90^\circ-\theta=15^\circ$$
$$\theta=90^\circ-15^\circ=75^\circ$$
Therefore $\cos75^\circ$ is the answer.
