A

$2 f \left(\cos x\right) f ' \left(\cos x\right)$

B

$- 2 f \left(\cos x\right) f ' \left(\cos x\right)$

C

$2 f \left(\cos x\right) f ' \left(\cos x\right) \sin x$

D

$- 2 f \left(\cos x\right) f ' \left(\cos x\right) \sin x$