(D) We use the formula $\cos \theta \cos(2\theta) \cos(4\theta) = \frac{\sin(8\theta)}{8\sin \theta}$.Here,$\theta = 20^\circ$.So,$\cos 20^\circ \cos

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(D) We use the formula $\cos \theta \cos(2\theta) \cos(4\theta) = \frac{\sin(8\theta)}{8\sin \theta}$.Here,$\theta = 20^\circ$.So,$\cos 20^\circ \cos 40^\circ \cos 80^\circ = \frac{\sin(8 \times 20^\circ)}{8 \sin 20^\circ}$.$= \frac{\sin 160^\circ}{8 \sin 20^\circ}$.Since $\sin 160^\circ = \sin(180^\circ - 20^\circ) = \sin 20^\circ$,$= \frac{\sin 20^\circ}{8 \sin 20^\circ} = \frac{1}{8}$.