Which of the following is incorrect?

(a) $\sin x=-\frac{1}{5}$ is correct as $-1 \leq \sin x \leq 1$

(b) $\cos x=1$ is correct as $-1 \leq \cos x \leq 1$

(c) $\sec x=\frac{1}{2}$ is not correct as sec $x \in(-\infty,-1] \cup[1, \infty)$

(d) $\tan x=20$ is correct as $\tan x$ can take any real value.

Hence, the correct answer is option C.