\[\boxed{\mathbf{84.}}\]
\[\textbf{а)}\ y = \lg{\cos x}\]
\[\textbf{б)}\ y = \sqrt{ctgx + 1}\]
\[\textbf{в)}\ y = 2^{\text{tgx}}\]
\[\textbf{г)}\ y = \left( \log_{2}\sqrt{x - 1} \right)^{3}\]