Answer
6 is matched with C to build an identity.
Work Step by Step
According to the given identity, we have that $$\sin(\frac{\pi}{2}-x)=\cos x$$
We need do some transformations with the sixth expression in I to bring it back to identity: $$\sin(x-\frac{\pi}{2})$$ $$=\sin[-(\frac{\pi}{2}-x)]$$
Yet we already know that $\sin(-X)=-\sin X$. Therefore, $$=-\sin(\frac{\pi}{2}-x)$$ $$=-\cos x$$
Now we match with the expressions in II. We find that the sixth expression in I matches the expression C in II.
Therefore, we would match 6 with C to have an identity.
