In a trap$-$door one$-$way function, what condition makes the computation of $x=f_k^{-1}(Y)$ infeasible?

Select one:

a$.$ If $Y$ and $k$ are known

b$.$ If $Y$ is known but $k$ is not known

c$.$ If $k$ is known but $x$ is not known

d$.$ If $x$ and $k$ are known