Explanation

(b)
Given, $f(x) = \log_{2}\left( \log_{3}\left( \log_{4}x \right) \right)$
We know, $\log_{a}x$ is defined, if $x > 0$
For $f(x)$ to be defined.
${\log_{3}\log_{4}}{x > 0,\log_{4}{x > 0}}\$ and $x > 0$
$\Rightarrow \ \ \ \log_{4}{x > 3^{0} = 1,x > 4^{0} = 1and\ x > 0}$
$\Rightarrow \ \ \ x > 4,\ x > 1$ and $x > 0$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ x > 4$