Which of the following functions is inverse of itself in its domain?

Since $fof(x)=f(f(x))=f\left(\frac{1-x}{1+x}\right)=\frac{1-\frac{1-x}{1+x}}{1+\frac{1-x}{1+x}}=x,∀\,x$

$∴ fof = I$ ⇒ f is the inverse of itself.