If $x \neq(2 n+1) \frac{\pi}{4}$, then the general solution of $\cos x+\cos 3 x=\sin x+\sin 3 x$ is

• A

  $\mathrm{n} \pi+\frac{\pi}{8}$
• B

  $\mathrm{n} \pi \pm \frac{\pi}{8}$
• C

  $\frac{n \pi}{2} \pm \frac{\pi}{8}$
• Correct answer
  D

  $\frac{n \pi}{2}+\frac{\pi}{8}$

Hence, the correct answer is option: 'D'.