A \( \frac{1}{4}\sin\left(\frac{n\pi}{2} + 3\right) \)
B \( \frac{1}{4}\sin\left(\frac{n\pi}{2} + 4\right) \)
C \( \frac{1}{4}\sin\left(\frac{n\pi}{2} + 5\right) \)
D \( \frac{1}{4}\left[3 \sin\left(\frac{n\pi}{2} + x\right) - 3^n \sin\left(\frac{n\pi}{2} + 3x\right)\right] \)