If \(f(x)=\left\{\begin{array}\text{x} & when\,x\,is\,rational \\ 1-x & when\,x\,is\,irrational\end{array}\right.\), then fof (x) is given as

x

\(fof(x)=\left\{\begin{array}\text{f(x)} & when\,f(x)\,is\,rational \\ 1-f(x) & when\,f(x)\,is\,irrational\end{array}\right.\)

$=\left\{\begin{array}\text{x} & when\,x\,is\,rational \\ 1-(1-x) & when\,x\,is\,irrational\end{array}\right.=x$

Hence (B) is the correct answer.