(c) $\cos 20° > \cos 70°$
As the angle increases from 0° to 90°, cosine decreases. So $\cos 20° > \cos 70°$. (sin and tan increase, making options a, b, d false.)
Key rule: sin and tan are increasing functions on (0°, 90°), while cos is a decreasing function. So a smaller angle gives a larger cosine value — hence $\cos 20° > \cos 70°$ is the only true statement. Eliminate the others: sin 20° < sin 70° (sin increases), cos 20° > sin 20° only when angle < 45° but that's not what option (b) says — sin 20° < cos 20°, and tan 20° < tan 70° (tan increases). Examiners expect you to recall increasing/decreasing behaviour of trig ratios.