Which of the Following Is Not a Valid Completion of the Double-Angle

Multiple Choice

Which of the following is not a valid completion of the double-angle identity for $\cos 2 \theta ?$

A) $2 \cos ^{2} a-1$
B) $1-2 \sin ^{2} \theta$
C) $\sin ^{2} \theta-\cos ^{2} \theta$
D) $\cos ^{2} \theta-\sin ^{2} \theta$