$a)f\left(x\right)=\left|\mathrm{tg}x\right|+1$

$D_f=\left\{\frac{k\pi }{2}\right\},k\in C$  

$b)f\left(x\right)=\left|\mathrm{tg}\left(x+\frac{\pi }{2}\right)\right|$  

$D_f=\left\{k\pi \right),k\in C$  

$c)f\left(x\right)=-2\left|\mathrm{tg}\left(x-\frac{\pi }{4}\right)\right|$

$D_f=\left\{-\frac{\pi }{4}+k\pi \right\},k\in C$