Answer
For an identity to be made, 3 should be matched with E.
Work Step by Step
According to the given identity, we have that $$\cos(x-y)=\cos x\cos y+\sin x\sin y$$
Now apply to the case in third expression: $$\cos(\frac{\pi}{2}-x)=\cos(\frac{\pi}{2})\cos x+\sin(\frac{\pi}{2})\sin x$$ $$=0\times\cos x+1\times\sin x$$ $$=\sin x$$
Now we match with the expressions in II. We find that the third expression in I matches the expression E in II.
Therefore, we would match 3 with E to have an identity.